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Section 2: Environmental Dynamics and Drivers

Overview

In the second section of the course, you will begin to explore the interaction between our human food systems and natural earth systems, including soil and water resources and how those resources influence the selection of which crops a farmer grows. Soil and water are two key ingredients are critical to growing food. Module 4 explores why water is necessary to grow food, where that water comes from, either precipitation or irrigation, and the impacts that our food production system has on Earth's water resources. Module 5 focuses on the other essential resource for food systems, soil and the nutrients found in soil. In this module, you will explore how soil resources can be degraded and how soil management can help protect soils and key soil nutrients, nitrogen (N) and phosphorus (P). Module 6 is an introduction to crops. You will explore how climate, including temperature and precipitation, and soil resources influence crop plant selection, how crops are classified and what other factors influence crop selection.

At the end of this section, you'll explore each of these topics in your capstone region in Stage 2 of the capstone assignment. Your goal at this stage is to identify the water resources available in your capstone region by looking at climate maps, to identify the quality of the soil resources and soil and nutrient management practices in your capstone region, and to explore the types of crops grown in your capstone region.

Modules

  • Module 4: Food and Water
  • Module 5: Soils as a key resource for food systems
  • Module 6: Crops
  • Capstone Stage 2

Section Goals

Upon completion of Section 2 students will be able to:

  • Identify soil nutrients and soil function as key natural system factor in food production.
  • Distinguish between pre-existing aspects of biogeochemical cycling and human-induced processes that affect biogeochemical cycling.
  • Attribute different soil fertility outcomes in food systems to the coupled natural and human factors and feedbacks that produce them.
  • Analyze the relationship between climate, availability of water resources, irrigation, and agricultural food production.
  • Examine their water footprints and the virtual water embedded in agricultural food products.
  • Summarize the major impacts of agriculture on both quality and quantity of water resources.
  • Describe key features of categories of crop plants and how they are adapted to environmental and ecological factors.
  • Explain how soil and climatic features determine what crops can be produced in a location, and how humans may alter an environment for crop production.
  • Explain how both environmental and socio-economic factors contribute to crop plant selection (coupled human-nature systems).
  • Outline the basic science behind human-induced climate change and the contribution from agriculture.
  • Compare various potential impacts of climate change on our global and local food systems.
  • Select strategies that enhance the resilience of food systems in the face of a changing climate.

Section Objectives

In order to reach these goals, we have established the following learning objectives for student learning. Upon completion of the modules within Section 2, you will be able to:

  • Describe the basic properties of soil that distinguish it from mere "dirt".
  • Explain how soil serves as a medium for plant growth.
  • Explain how the five soil-forming factors interact to produce soils.
  • Explain the term "biogeochemical cycling".
  • Explain common limiting factors to plant growth that limit food production around the world.
  • Explain how nutrient and carbon depletion from soils and soil erosion create conditions of low food productivity.
  • Assess how farming practices affect soil fertility.
  • Analyze modern fertilizer use as the emergence of a strong human system impact on and participation in natural biogeochemical cycling that addresses nutrient limitation but can create nutrient pollution.
  • Analyze how natural/human system feedbacks operate to limit the actions of poorer food producers around the world.
  • Incorporate sustainability challenges related to soil nutrient management into an analysis of food systems.
  • Explain the relationships between evapotranspiration (ET), climate, and crop consumptive use.
  • Relate the spatial distribution of precipitation and ET rates to where food can be grown with and without irrigation.
  • Estimate their water consumption in the food they eat using the concepts of virtual water and water footprints.
  • Attribute major water pollutants to appropriate agricultural sources.
  • Describe the major impacts of agricultural diversions on the Colorado River.
  • Relate nutrient loading from fertilizer use to the dead zone in the Gulf of Mexico.
  • Define annual and perennial crops and list some examples of annual and perennial crops.
  • Discuss why annual or perennial crops are cultivated in high resource or resource-limited environments.
  • Explain some ways that farmers alter the environment to produce annual or perennial crops.
  • Explain some of the advantages and disadvantages of producing annual and perennial crops.
  • Name some major crop plant families.
  • Explain the nutrient significance of legumes.
  • Describe some ways that plants are classified into types including plant families, temperature adaptation, and photosynthetic pathways.
  • Describe key plant physiological processes and how climate change may influence crop plant growth and yield.
  • Describe some of the major agricultural crops and the agroecological (both ecological and socioeconomic) factors that influence what crops farmers produce.
  • Describe examples of the conflict of growing food crops for food versus biofuel.
  • Identify climate variables that affect agriculture.
  • Explain possible climate change impacts on crops.
  • Summarize the mechanisms of human-induced climate change.
  • Explain the role of food systems in contributing to climate change.
  • Discuss how climate change impacts food production and yield.
  • Evaluate how farmers adapt to climate change.
  • Differentiate impacts of climate change on climate variables in different regions.

Module 4: Food and Water

Overview

Water is an essential element in growing the food we eat. Also, the growing of our food has an effect on Earth's water resources as agricultural runoff contributes to pollution and diversions for irrigation affect streamflow and deplete aquifers. In this module, we'll look at how water is a critical element in the production of food. We'll also explore some of the impacts that our food systems has on both the quality and quantity of our water resources.

Plants can't grow without water and in this module, we explore how plants use water and where that water comes from. Have you ever considered that fact that you eat a lot of water? All of the food you eat required water to grow, process, and transport. How much water did it take to make grow feed for the cattle that ultimately became the hamburger you had for lunch this week? Or to feed the chicken that laid the egg for your breakfast? Or to grow the coffee beans for your morning latte? Water is an essential component of our food system!

Goals and Learning Objectives

Goals

  • Analyze the relationships between climate, availability of water resources, irrigation, and agricultural food production.
  • Examine their water footprints and the virtual water embedded in agricultural food products.
  • Summarize the major impacts of agriculture on both quality and quantity of water resources.

Learning Objectives

After completing this module, students will be able to:

  • Explain the relationships between evapotranspiration (ET), climate, and crop consumptive use.
  • Describe the major impacts of agricultural diversions on the Colorado River.
  • Relate the spatial distribution of precipitation and ET rates to where food can be grown with and without irrigation.
  • Relate nutrient loading from fertilizer use to the dead zone in the Gulf of Mexico.
  • Attribute major water pollutants to appropriate agricultural sources.
  • Estimate their water consumption in the food you eat using the concepts of virtual water and water footprints.

Assignments

Module 4 Roadmap

Detailed instructions for completing assessments are provided with each module.

Assignment Location
Module 4 Roadmap
To Read
  1. Materials on the course website.
  2. EPA Fact Sheet on Agricultural Runoff: Protecting Water Quality from Agricultural Runoff
  1. You are on the course website now.
  2. Online: Protecting Water Quality from Agricultural Runoff [1]
To Do
  1. Formative Assessment: Turning Water into Food
  2. Summative Assessment: Kansas Farm Case Study
  3. Participate in the Discussion
  4. Take Module Quiz
  1. In course content: Formative Assessment [2]; then take the formative quiz in Canvas
  2. In course content: Summative Assessment [3]; then take the summative quiz in Canvas
  3. In Canvas
  4. In Canvas

Questions?

If you prefer to use email:

If you have any questions, please send them through Canvas e-mail. We will check daily to respond. If your question is one that is relevant to the entire class, we may respond to the entire class rather than individually.

If you prefer to use the discussion forums:

If you have any questions, please post them to the discussion forum in Canvas. We will check that discussion forum daily to respond. While you are there, feel free to post your own responses if you, too, are able to help out a classmate.

Module 4.1: Water Resources and Food Production

Introduction

How much water do you eat? Water is essential for food production. In this unit, you will learn about water as an essential ingredient to grow the food that we eat, including plants and animal products. The concepts of photosynthesis, evapotranspiration, and crop consumptive water use are introduced followed by an overview of the spatial variability of precipitation and the resulting need for irrigation. The final activity will introduce you to virtual water embedded in the food you eat and your water footprint.

The short animated video that follows was produced by the United Nations' Water group for World Water Day and illustrates how much water is embedded in a few different food products. The numbers are given in liters, so it's helpful to remember that there are 3.8 liters per gallon. A liter is a little bigger than a quart. In this module, we'll look at why it takes so much water to produce food and you'll estimate how much water you eat.

Video: All You Eat (0:49)

Click for video transcript.
This video has music only, no voice. Words on the screen read: 

World water day 2012

Why is water so important to our food security?

Your bread: 650 liters

Your milk: 200 liters

Your eggs: 135 liters

Your steal: 7000 liters

Your vegetables: 13 liters

Your burger: 2400 liters

ALL YOU EAT NEEDS WATER TO GROW

Agriculture accounts for 70% of our total water use

So. Now. You. Know. Why.

By: Faowater

If you do not see the video above, please go to YouTube [4] to watch it.

Evaporation and Climate

In order to understand why growing food uses so much water, we need to explore the process of evaporation. Evaporation is a hydrologic process that we're all quite familiar with, even if you aren't aware of it. Think about hanging clothes out to dry on the clothesline, or blow drying your hair. Both of those involve the movement of water from its liquid form to its vapor or gaseous form that we call water vapor, or in other words, both involve the evaporation of water.

In what weather conditions do your clothes dry faster? A hot, dry, windy day, or a cool, cloudy, rainy day? Why do you use a blow drier to dry your hair? Water evaporates faster if the temperature is higher, the air is dry, and if there's wind. The same is true outside in the natural environment. Evaporation rates are generally higher in hot, dry and windy climates.

The rate at which water evaporates from any surface, whether from a lake's surface or through the stomata on a plant's leaf, is influenced by climatic and weather conditions, which include the solar radiation, temperature, relative humidity and wind (and other meteorological factors). Evaporation rates are higher at higher temperatures because as temperature increases, the amount of energy necessary for evaporation decreases. In sunny, warm weather the loss of water by evaporation is greater than in cloudy and cool weather. Humidity, or water vapor content of the air, also has an effect on evaporation. The lower the relative humidity, the drier the air, and the higher the evaporation rate. The more humid the air, the closer the air is to saturation, and less evaporation can occur. Also, warm air can “hold” a higher concentration of water vapor, so you can think of there being more room for more water vapor to be stored in warmer air than in colder air. Wind moving over a water or land surface can also carry away water vapor, essentially drying the air, which leads to increased evaporation rates. So, sunny, hot, dry, windy conditions produce higher evaporation rates. We will see that the same factors - temperature, humidity, and wind - will affect how much water plants use, which contributes to how much water we use to produce our food!

Evaporation requires a lot of energy and that energy is provided by solar radiation. The maps below (Figure 4.1.1) illustrate the spatial patterns of solar radiation and of annual evaporation rates in the United States. Notice how the amount of solar radiation available for evaporation varies across the US. Solar radiation also varies with the season and weather conditions. Note that annual evaporation rates are given in inches per year. For example, Denver, Colorado in the lake evaporation map is right on the line between the 30-40 inches and 40-50 inches per year of lake evaporation, so let's say 40 inches per year. On average, if you had a swimming pool in Denver, and you never added water and it didn't rain into your pool, the water level in your pool would drop by 40 inches in a year. Explore the maps and answer the questions below.

Map of Mean daily solar radiation in the United States and Puerto RicoMap of Mean annual lake evaporation in the conterminous United States, 1946-55
Figure 4.1.1. a. Mean daily solar radiation in the United States and Puerto Rico and
b. Mean annual lake evaporation in the conterminous United States, 1946-55. Data not available for Alaska, Hawaii, and Puerto Rico.
Source: Data from U.S. Department of Commerce, 1968). From Hanson 1991.

Check Your Understanding

How are the patterns on the two maps above (Figure 4.1.1) similar? Which regions experience high solar radiation and which regions experience high evaporation rates?&

Click for answer.

Answer: Generally, the spatial patterns of solar radiation and lake evaporation in the US are similar as high solar radiation drives high evaporation. The southwestern region of the US has both high solar radiation and high evaporation rates.

How are the two maps different? What factors might contribute to the differences?

Click for answer.

Answer: Two main differences are:

  • The Rocky Mountain region where high elevations lead to cooler temperatures which lowers evaporation rates
  • The southeastern US where high humidity reduces evaporation.

Find your location on the maps. How much solar radiation does your location receive per year, and how much water would evaporate from a lake on average per year?

Click for answer.

Answer: Find the solar radiation and lake evaporation for your location using the maps below. Note that lake evaporation in Figure 4.1.1b is given in inches per year. Is this value higher than you expected?

Water is Essential for Food Production

Why do we need so much water for agriculture? Plants use a lot of water!

Plants need water to grow! Plants are about 80-95% water and need water for multiple reasons as they grow including for photosynthesis, for cooling, and to transport minerals and nutrients from the soil and into the plant.

"We can grow food without fossil fuels, but we cannot grow food without water."
Dr. Bruce Bugbee, Utah State University

We can't grow plants, including fruits, vegetables, and grains, without water. Plants provide food for both us and for the animals we eat. So, we also can't grow cows, chickens or pigs without water. Water is essential to growing corn as well as cows!

Agriculture is the world's greatest consumer of our water resources. Globally about 70% of human water use is for irrigation of crops. In arid regions, irrigation can comprise more than 80% of a region's water consumption.

The movement of water from the soil into a plant's roots and through the plant is driven by an evaporative process called transpiration. Transpiration is just evaporation of water through tiny holes in a plant's leaves called stomata. Transpiration is a very important process in the growth and development of a plant.

Water is an essential input into the photosynthesis reaction (Figure 4.1.2), which converts sunlight, carbon dioxide, and water into carbohydrates that we and other animals can eat for energy. Also, as the water vapor moves out of the plant's stomata via transpiration (Figure 4.1.2), the carbon dioxide can enter the plant. The transpiration of water vapor out of the open stomata allows carbon dioxide (another essential component of photosynthesis) to move into the plant. Transpiration also cools the plant and creates an upward movement of water through the plant. The figure below (Figure 4.1.2) shows the photosynthesis reaction and the movement of water out of the plant's stomata via transpiration.

As water transpires or evaporates through the plant's stomata, water is pumped up from the soil through the roots and into the plant. That water carries with it, minerals and nutrients from the soil that are essential for plant growth. We'll talk quite a bit more about nutrients later in this module and future modules.

Schematic of Photosynthesis and transpiration
Figure 4.1.2 Photosynthesis and transpiration
Credit: Wikimedia Commons: Photosynthesis [5] (Creative Commons CC BY-SA 3.0 [6])
Click for a text description of the image

This drawing shows the sunlight shining down on a flower. The roots of the flower are in the soil and there is water in the soil. Carbon dioxide is going into the flower. Water vapor and oxygen are being released from the flower (Transpiration). The chemical formula for photosynthesis is shown as 6 CO2 (Carbon Dioxide) + 6 H2O (Water) an arrow representing light leads to C6H12O6 (Sugar) + 6 O2 (Oxygen).

Evapotranspiration and Crop Water Use

How much water does a crop need?

The amount of water that a crop uses includes the water that is transpired by the plant and the water that is stored in the tissue of the plant from the process of photosynthesis. The water stored in the plant's tissue is a tiny fraction (<5%) of the total amount of water used by the plant. So, the water use of a crop is considered to be equal to the water transpired or evaporated by the plant.

Since a majority of the water used by the crop is the water that is transpired by the plant, we measure the water use of a plant or crop as the rate of evapotranspiration or ET, which is the process by which liquid water moves from the soil or plants to vapor form in the atmosphere. ET is comprised of two evaporative processes, as illustrated in figure 4.1.3 below: evaporation of water from soil and transpiration of water from plants' leaves. ET is an important part of the hydrologic cycle as it is the pathway by which water moves from the earth's surface into the atmosphere.

Remember, evaporation rates are affected by solar radiation, temperature, relative humidity, and the wind. ET, which includes evaporation from soils and transpiration from plants, is also evaporative, so the ET rate is also affected by solar radiation, temperature, relative humidity, and the wind. This tells us that the crop water use will also be affected by solar radiation, temperature, relative humidity, and the wind! More water evaporates from plants and soils in conditions of higher air temperature, low humidity, strong solar energy and strong wind speeds.

The transpiration portion of ET gets a little more complicated because the structure, age, and health of the plant, as well as other plant factors, can also affect the rate of transpiration. For example, desert plants are adapted to transpire at slower rates than plants adapted for more humid environments. Some desert plants keep their stomata closed during the day to reduce transpiration during the heat of a dry desert day. Plant adaptations to conserve moisture include wilting to reduce transpiration. Also, small leaves, silvery reflective leaves, and hairy leaves all reduce transpiration by reducing evaporation.

In summary, the amount of water that a crop needs is measured by the ET rate of a crop. The ET rate includes water that is transpired or evaporated through the plant. And, the ET rate varies depending on climatic conditions, the plant characteristics, and the soil conditions.

Schematic of Evapotranspiration
Figure 4.1.3. Evapotranspiration includes evaporation from soil and transpiration from leaves.
Credit: Figure drawn by Gigi Richard, adapted from Bates, R.L. & J.A.Jackson, Glossary of Geology, Second Edition, American Geology Institute, 1980
Click for a text description of the image

Diagram of Evapotranspiration. At the bottom is soil and below that, available soil water. In the soil are two plants with roots extending into the soil water. There are lines coming up from the soil representing evaporation from the soil. Lines from the plants represent transpiration from leaves. There is a line drawn around all of this, with the sun outside and humidity and temperature flowing in. An arrow from transpiration and evaporation leads to evapotranspiration.

Crop water use varies

If the ET rate of a crop determines the water use of that crop, we could expect water use of a single crop to vary in similar spatial patterns to evaporation rates. For example, if evaporation rates are very high in Arizona because the hot, dry climate, you would expect ET rates to be higher for a given crop in that climate. ET is measured by the average depth of water that the crop uses, which is a function of the plant and of the weather conditions in the area. In cool, wet conditions, the plant will require less water, but under hot, dry conditions, the same plant will require more water.

Figure 4.1.4 shows a range of typical water use for crops in California. The graph shows how much water needs to be applied as irrigation to grow different crops. Notice how some crops, like alfalfa, almonds, pistachios, rice and pasture grass can require four feet or more of water application. Other crops, like grapes, beans, and grains only require about between one and two feet of water.

If we moved the plants in Figure 4.1.4 to a cooler and more humid climate, the rate of evaporation would be less and the crop water demand would decline as well. In a hot dry climate, you need to apply more water to the plant to keep it healthy and growing because more water is evaporating from both the soil and through the stomata on the plants’ leaves, so the plant is pulling more water out of the soil via its roots to replace the water transpiring from its leaves.

Graph of  Water use of California crops, as measured by water application depth
Figure 4.1.4. Water use of California crops, as measured by water application depth. Note: The range shown reflects the first and third quartile of the water application depth for the period 1998-2010, and the white line within that range reflects the median application depth during that period.
Credit: California Agricultural Water Use: Key Background Information [7], by Heather Cooley, Pacific Institute.

Water Sources for Crops

Where do plants get their water?

The source of water for most land plants is precipitation that infiltrates or soaks into the soil, but precipitation varies dramatically geographically. For example, we know that Florida gets a lot more precipitation per year than Arizona. Figure 4.1.5 below shows the average annual precipitation across the United States and around the globe. Notice on the map of the U.S. that the dark orange colors represent areas that get less than ten inches of precipitation per year. And, the darkest green to blue regions receive more than 100 inches or more than eight feet of precipitation per year!

Climate, including the temperature of a region and the amount of precipitation, plays an important role in determining what types of plants can grow in a particular area. Think about what types of plants you might see in a high water resource region versus a low water resource region. A low resource region with respect to water receives lower precipitation, so would have desert-like vegetation, whereas a higher resource region for water would have lusher native vegetation, such as the forests of the eastern US.

Regions that receive enough precipitation to grow crops without irrigation (i.e., those areas shaded green on the map below) would be considered high resource areas with respect to water. A high resource region is more likely to be a more resilient food production region. In contrast, a low resource region with respect to water would be regions on the map below in the orange shaded colors. In these regions, extra effort is needed to provide enough water for crops, such as through the development of an irrigation system.

Compare the crop water use values in Figure 4.1.6 with the average annual precipitation in Figure 4.1.5 and you'll see that there are parts of the US where there isn't enough precipitation to grow many crops. In fact, there is a rough line running down the center of the US at about the 100th meridian that separates regions that get more than about 20 inches of rain per year from regions that get less than 20 inches of rain per year. On the map in Figure 4.1.5, this line is evident between the orange colored areas and the green colored areas. Generally, west of the 100th meridian there is insufficient precipitation to grow many crops. If a crop's consumptive water use or ET is greater than the amount of precipitation, then irrigation of the crop is necessary to achieve high yields.

US Map shows average annual precipitation 1961-1990
Global Annual Average Precipitation Map
Figure 4.1.5. a) Average annual precipitation in the United States 1961-1990 and b) Mean annual Global Precipitation (1961-1990)
Credit a) USGS: The National Map [8] and b)Terrascope: Rainwater Harvesting [9]
Graph of water use of California crops
Figure 4.1.6. Water use of California crops, as measured by water application depth. Note: The range shown reflects the first and third quartile of the water application depth for the period 1998-2010, and the white line within that range reflects the median application depth during that period.
Credit: California Agriculture Water Use: Key Background Information [7] by Heather Cooley, Pacific Institute.

How can we grow crops when there is insufficient precipitation?

In regions where precipitation is insufficient to grow crops, farmers turn to other sources of water to irrigate their crops. Irrigation is the artificial application of water to the soil to assist in growing of agricultural crops and other vegetation in dry areas and during periods of inadequate rainfall. These sources of water can be from either surface or groundwater. Surface water sources include rivers and lakes, and diversion of water from surface water sources often requires dams and networks of irrigation canals, ditches, and pipelines. These diversions structures and the resulting depletion in river flow can have significant impacts on our rivers systems, which will be covered in the next part of this module. Pumping of water for irrigation from aquifers also has impacts, which are also discussed in the next part of this module.

Water use for irrigation comprised about 80-90 percent of U.S. consumptive water use in 2005, with about three-quarters of the irrigated acreage being in the western-most contiguous states (from USDA Economic Research Service [10]). For example, in the state of Colorado, irrigation comprised 89% of total water withdrawals in 2010 (Figure 4.1.7). Irrigated agriculture is also very important economically, accounting for 55 percent of the total value of crop sales in the US in 2007 (from USDA Economic Research Service [10]). Globally only about 18 percent of cropland is irrigated, but that land produces 40 percent of the world's food and about 50 percent by value (Jones 2010).

Pie graph of total water withdrawals by water-use category in 2010 in Colorado
Figure 4.1.7. Colorado Total water withdrawals by water-use category
Credit: Data from Maupin, M. A., Kenny, J.F., Hutson, S.S., Lovelace, J.K., Barber, N.L., and Linsey, K.S., 2014, Estimated use of water in the United States in 2010: U.S. Geological Survey Circular 1405, 56 pp. [11]
Click for a text description of the image
A pie chart of Colorado total water withdrawals by water-use category in 2010 shows that almost 89% percent of water withdrawals were from irrigation. The remaining percentages are as follows (approximately): 8% public water supply, 1.18% industrial, 1.11% aquaculture, .70% thermoelectric power, .34% domestic fresh, .33% livestock, .26% mining.

Activate Your Learning

In this activity, you will employ geoscience ways of thinking and skills (spatial thinking and interpretation of the spatial data to characterize specific regions for the geographic facility). Compare the evaporation rates in Figure 4.1.1 and the average annual precipitation in Figure 4.1.5 and discuss where you would expect irrigation demands to be highest. Consider the precipitation distribution for just the state of California from south to north. Which regions would be considered low resource regions from a water perspective? Which regions are high resource regions?

Click for answer.

Answer: The regions with highest irrigation demand are the areas with low precipitation and high evaporation, which includes the southwestern US, and most of the western states, except for the coastal areas of Washington, Oregon, and northern California. Precipitation in California varies from only 5 inches per year in the deserts of the southern part of the state up to about 70 inches in some of the mountainous and northern coastal regions of the state.

From a water perspective, regions with higher annual precipitation are high resource regions, and regions with lower annual precipitation are low resource regions. In general, the eastern US is a higher resource region from a water perspective, and the desert southwest is a very low water resource region.

Identify the locations of the Imperial and Central Valleys in California, which are major agricultural regions. What is the average annual precipitation that falls in these regions? Which crops from Figure 4.1.6 c

Click for answer.

Answer: The precipitation rates in the Central and Imperial valleys are generally less than 15 inches per year and in some areas 5 inches and less per year. None of the crops in Figure 4.1.6 could be grown in those regions without additional water. At 15 inches of precipitation per year, some areas of the central valley might be able to grow safflower and some grains, but it depends on when the precipitation falls and if it is during the crops' growing season.

What alternatives do farmers have when faced with insufficient precipitation?

Click for answer.

Answer: When faced with insufficient precipitation for crop growth, farmers can divert water from surface or groundwater for irrigation.
map of mean annual lake evaporation in the conterminous United States, 1946-55.
Figure 4.1.1. Mean annual lake evaporation in the conterminous United States, 1946-55.
Credit: Data from U.S. Department of Commerce, 1968. From Hanson 1991.

Irrigation Efficiency

The amount of water used for irrigation varies depending on the climate and on the crop being grown, and it also depends on the irrigation technique used. Just like in your garden or home landscaping there are more or less efficient sprinklers. In many parts of the world flood or surface, irrigation is still used where water flows across a field and soaks into the soil.

Surface or flood irrigation is the least efficient manner of irrigation. When a field is flooded, more water than is needed by the plant is applied to the field and water evaporates, seeps into the ground and percolates down to the groundwater, where it can be out of reach of the plant's roots. Another problem with flood irrigation is that the water is not always applied evenly to all plants. Some plants might get too much water, and others get too little. On the other hand, flood irrigation tends to use the least energy of any irrigation system.

Furrow irrigation (Figure 4.1.8) is another type of surface irrigation in which water is directed through gated pipe or siphon tubes into furrows between rows of plants. When using furrow irrigation, water is lost to surface runoff, groundwater, and evaporation, and it can be challenging to get water evenly to an entire field.

furrow irrigation systemFurrow irrigation of an onion field in the Uncompahgre Valley, CO.
Figure 4.1.8. Furrow irrigation of an onion field in the Uncompahgre Valley, CO.
Credits: Perry Cabot

More efficient methods of irrigation include drip irrigation (Figure 4.1.9) sprinklers (such as center pivots, Figure 4.1.10), and micro-spray (Figure 4.1.11) irrigation. All of these methods, while more efficient, also require significant investments in equipment, pipes, infrastructure (e.g., pumps Figure 4.1.9) and energy. In addition to the high cost, some soil types, irrigation networks, field sizes and crops pose greater challenges to implementation of more efficient methods of irrigation. For example, in the Grand Valley of western Colorado, the irrigation network is entirely gravity-fed, meaning that farmers can easily flood and furrow irrigate without the use of pumps. In addition, the fields are small and the soils are very clayey, all of which make using center pivots for row crops particularly challenging and expensive. But, in the same valley, the peach orchards have successfully used micro-spray and drip systems. A major advantage of more efficient irrigation in addition to reduced water consumption is that crop yields are often higher because the water can be applied more directly to the plant when water is needed.

drip irrigation system
Figure 4.1.9. Filtration and pumps for a drip irrigation system for onion and bean crops in the Uncompahgre Valley, CO.
Credit: Gigi Richard
Center pivot sprinkler irrigation on an alfalfa crop in the San Luis Valley, COa hay crop for cattle feed in the Uncompahgre Valley, CO
Figure 4.1.10. a) Center pivot sprinkler irrigation on an alfalfa crop in the San Luis Valley, CO and b) a hay crop for cattle feed in the Uncompahgre Valley, CO.
Credit: Gigi Richard
Micro-spray irrigation at a peach orchard in the Grand Valley, CO.
Figure 4.1.11. Micro-spray irrigation at a peach orchard in the Grand Valley, CO.
Credit: Gigi Richard

Activate Your Learning

Table 4.1.1 below presents data on the top 15 irrigated states in the United States. You can see how many acres of land are irrigated in each state, and how much water is used for irrigation of both surface water and groundwater. Consider the relationship between the amount of irrigated land in a state, the type of irrigation used and the amount of water used. Answer the questions below.

An acre-foot is a unit of measure for large volumes of water and is the volume of water required to cover one acre of land to a depth of one foot (325,851 gallons). Imagine a football field, including the end zones, one foot deep in water.

Table 4.1.1. Top 15 Irrigated States, 2010
Data from U.S. Geological Survey, 2014, Estimated Use of Water in the United States in 2010, Circular 1405, Washington, D.C., U.S. Department of Interior
State Irrigated Land (in thousand acres)
by type of irrigation
Surface Water Withdrawals Groundwater Withdrawals Total Irrigation Withdrawals
Sprinkler Micro-irrigation Surface Total Thousand acre-feet per year % of irrigation water from surface water Thousand acre-feet per year % of irrigation water from groundwater Thousand acre-feet per year Percent of total water withdrawals
used for irrigation
California 1790 2890 5670 10400 16100 62% 9740 38% 25840 61%
Idaho 2420 4.57 1180 3600 11500 73% 4280 27% 15780 82%
Colorado 1410 0.2 1930 3340 9440 87% 1450 13% 10890 88%
Arkansas 518 0 4150 4670 1500 15% 8270 85% 9770 77%
Montana 753 0.64 886 1640 7880 98% 142 2% 8022 94%
Texas 3770 244 1910 5920 1940 25% 5710 75% 7650 27%
Nebraska 6370 0.57 2360 8730 1520 24% 4820 76% 6340 70%
Oregon 1210 97 594 1900 3750 64% 2140 36% 5890 78%
Arizona 195 28.1 770 993 3220 63% 1900 37% 5120 75%
Wyoming 184 4.12 892 1080 4410 90% 490 10% 4900 93%
Utah 625 1.45 710 1340 3060 85% 554 15% 3614 72%
Washington 1270 86.1 221 1580 2630 75% 894 25% 3524 63%
Kansas 2840 18.1 217 3080 179 5% 3230 95% 3409 76%
Florida 548 712 731 1990 1500 46% 1770 54% 3270 20%
New Mexico 461 19.6 397 878 1640 54% 1390 46% 3030 86%

Do the states that use the most water also irrigate the most land? Which states are an exception?

Click for answer.

Answer: Idaho and Colorado use the second and third most water, but irrigate considerably less land than four other states. Nebraska irrigates more than twice as much land with less than half of the water that Idaho uses and about 2/3 of the water the Colorado uses.

Compare the data for Nebraska with Idaho. Nebraska's water withdrawals are much lower for a larger acreage of land than Idaho. What is the major source of Nebraska's irrigation water? Surface or ground water? And, which type of irrigation is used?

Click for answer.

Answer: Groundwater and center pivot sprinklers are common in Nebraska. In Idaho, by contrast, gravity-driven, surface-water irrigation is more common. Differences in application efficiencies account for wide variation in irrigation water withdrawals between regions.

What are two reasons, in addition to differences in irrigation efficiencies, that a state might use more water to irrigate less land?

Click for answer.

Answer: Difference in climate, that is temperature and humidity, can influence evaporation rates, and therefore affect crop consumption. Also, different plants consume different quantities of water, so irrigation needs vary depending on which crops are grown.

Virtual Water

How much water do you eat?

Water is essential to growing food and every bite of food we consume required water to grow, process and transport. The water necessary to grow, process and transport food is often referred to as virtual water or embedded water. Virtual water is the entire amount of water required to produce all of the products we use, including our mobile phones and cotton t-shirts. But a global assessment of virtual water reveals that the majority of water that we consume is in the food we eat. If we total up all of the virtual water embedded in everything we use and eat, we can estimate our total water footprint. Water footprints can be used to provide insights into how much water is used every day in all of our activities including producing our food. For example, Figure 4.1.12 shows the amount of water used per person around the globe associated with wheat consumption. When you eat food imported from another region, you are eating the water of that region. The apple from New Zealand, grapes from Chile and lettuce from California all required water to grow and by consuming those products you’re "eating" that virtual water. The concepts of virtual water and water footprints can be powerful tools for businesses and governments to understand their water-related risks and for planning purposes (water footprint network [12]).

world map Water footprint per capita related to consumption of wheat products 1996–2005
Figure 4.1.12. Water footprint per capita related to consumption of wheat products in the period 1996–2005.
Credit: Figure from Hoekstra, A.Y. and M.M. Mekonnen, 2012, The Water Footprint of Humanity, Proceedings of the National Academy of Sciences, vol. 109, no. 9 [13].

Check Your Understanding

Scroll through this infographic [14] explaining virtual water and answer the questions below.

How many liters of water do you "eat" every day?

Click for answer.

Answer: 3,496 liters

If there are 3.8 liters per gallon, how many 20-gallon aquariums is that?

Click for answer.

Answer: 3,496 liters is 920 gallons, which is 46 20-gallon aquariums of water that each of us "eats" per day.

What percentage of the total water consumed on average per person per day is associated with the production of the food we consume?

Click for answer.

Answer: 92% of the water we use is in our food

How many liters of water are needed to produce one kilogram of beef?

Click for answer.

Answer: 15,400 liters of water

How big would the wall of one-liter water bottles equivalent to 15,400 liters? Convert the size of the wall to feet. How big is it?

Click for answer.

Answer: The wall of one-liter water bottles would be 8 meters by 40 meters, or about 26 feet by 131 feet.

Based on the graph of the amount of water needed to produce different food products, what sort of diet would you conclude uses the least/most amount of water?

Click for answer.

Answer: The graph shows that in general plants require less water to produce per kilogram than animal products, except for coffee.

Formative Assessment

Turning Water into Food

Instructions

Please download the worksheet below for detailed instructions.

  • Download Module 4 Formative Assessment Activity Worksheet [15]

You will perform three activities in this assessment:

  1. Watch the video below, Turning water into food, and answer the questions on the worksheet as you watch the video
  2. Visit water footprint calculator [16] website, compare how your water footprint changes with varying levels of meat consumption, and answer questions on the worksheet. This portion of the assessment will be included in the weekly discussion and not included in the assessment quiz.
  3. Perform a comparison of the virtual water [17] embedded in different food products and answer questions on the worksheet.

Video: Turning water into food, Bruce Bugbee | TEDxUSU (16:32)

Click for video transcript.

This is my globe. I've had this globe for over thirty years to analyze the three-dimensional relationships among the continents and the water and the nations. Political boundaries have changed over the decades, but the fundamental relationships haven't changed. Like many globes like this, my globe has raised mountains. And I always thought those mountains were diminished on my globe so that it would make it easier to manufacture. Till one day, I looked up the height of Mount Everest and the diameter of the earth, and I got out my micrometers to check how much these were diminished. And to my amazement, they were embellished. They're considerably embellished. It was a very disturbing day for me. If the mountains are embellished, the oceans are similarly thin. And it turns out, if you take all the water on our blue planet, roll it up into a sphere, it comes out to the size of a ping-pong ball, a ping-pong ball!

But it doesn't stop there. Even though this is small, ninety-seven and a half percent of the water on our planet is saltwater. We can't drink it, we can't irrigate our crops with it. The two-and-a-half percent that's freshwater is the size of this small blue marble. Now, if I took this marble, I should put it up here on Greenland because 99% of our freshwater is frozen in glaciers, mostly Greenland and Antarctica. The 1% that's left is the size of a mustard seed. This mustard seed recycles and recycles and sustains life on the planet. We use about a gallon of water every day in the water we drink and in the food we eat. We use about another 20 gallons a day in washing things - washing our clothes and domestic use. But we use several hundred gallons of water every day, indirectly, in the food we eat. That amount dwarfs all the other uses.

In the United States, we dedicate 70% of our water resources to agriculture. I have spent much of my professional life studying how to improve water efficiency in agriculture and I'm joined in this effort by hundreds of colleagues around the world. The challenge is enormous. We can grow food without fossil fuels, but we cannot grow food without water. We think about our carbon footprint. We ought to be thinking about our water footprint, and even more importantly, we ought to be thinking about our global food print. The type of food that we eat has a bigger impact on the environment than the cars we drive. Eating a hamburger is equivalent in water use to taking an 80-minute shower.

To understand where water goes, it's useful to review the Earth's water cycle. As you can see from the globe, 70% of the planet surface is oceans, 30% is land. So the water cycle starts with one fundamental thing. The Sun shines on the oceans and water evaporates. This is an amazing process. All the salts are left behind. It’s distilled water coming out of the ocean. Anybody that has boiled a pot of water on their stove to dryness knows it takes an enormous amount of energy to evaporate water. The Sun does this every day for free, no fossil fuels, no fancy apparatus. Here's an amazing fact, more Sun shines on the earth in an hour than all of the people use in a year. So this water vapor from the ocean blows over to the land, falls on the land as rain, and soaks into the ground. It eventually runs back to the oceans in the rivers. We have a few thousand years of experience in ways to reuse this water. We built dams, we drill wells, we pump the water back up to the surface. It's still liquid water. The microbes in the soil have purified it. We drill more wells, we use it again. Eventually, it slips out of our grasp and runs back to the ocean. This is all liquid water. There’s two fates, the second one is shown here.

Now let's plant some seeds. The roots grow from the seeds and the water that used to go into the ocean is short cycled back to the roots of the plants. The Sun is hot. The same energy that falls on the ocean, falls on the plant leaves. To stay cool and hydrated, they evaporate water. It goes into the air, back to the ocean, falls as rain, and become saltwater again. We have far less control over this water vapor than we do over the liquid water that we can reuse. Without a continuous supply of water vapor, the plants dehydrate and food production stops. We irrigate to keep the plants hydrated. We have developed an amazing array of instruments to precisely tell when and how much to irrigate crops. They get just what they need, no more no less. In some older systems, 50% of the water evaporated from the soil surface and didn't get into the plants, went back to the ocean. In some of our modern systems we now have subsurface drip irrigation that can deliver 90% of the water right to the plants.

Every drop is precious. We call these efforts, more crop per drop. Even with our best efforts, we can't keep up, we can't grow the food we need to feed a hungry planet. So we access aquifers deep in the ground. These aquifers are called fossil aquifers because they formed a long time ago, they're difficult to recharge. We drill deep wells and pump that water up to the surface and irrigate the plants. These aquifers are being depleted far more rapidly than our fossil fuel reserves.

So how much crop can we get per drop? Let's take a look at these wheat plants over here. Wheat and rice are the biggest crops for direct human consumption on the planet. These two crops provide the vast majority of our calories. This wheat was developed here at Utah State University. My colleagues and I hybridized tall high yielding wheat with very short wheat to get a short high yielding wheat. We did this with NASA funding because we wanted to work with NASA to develop a life support system for space, that we could grow our own food in space independent of the planet. We've grown this wheat many times on the international space station and some of the astronauts turned out to be amazing photographers. This is a picture of this wheat at harvest on the International Space Station. That picture in the background is not a photo-shopped image of my globe. We grow this wheat hydroponically and if you haven't ever seen hydroponic wheat, here it is, the roots absorbing the water, going up to the tops of the plant. And if you’re a student in the lab, you know how much water this wheat takes every day. We developed this for a fast rate of development. This wheat is only three weeks old from transplanting to this tub. It'll be ready to harvest in five weeks. That's almost twice as fast as wheat in the field. Surprisingly, hydroponic wheat doesn't require any more water than field wheat. In fact, it’s often is less because there's no evaporation from the soil surface, there's no leaks, all the water goes through the plant. Even with perfect efficiency of every input, it still takes a hundred gallons of water to grow enough wheat to make a loaf of bread. A hundred gallons of water.

To emphasize this point, my students built this simulated hundred-gallon tank of water. If we put a faucet on this and dripped it into this tank into a plot big enough to grow that wheat, it would be empty about the time the wheat was ready to harvest. This greatly exceeds all the other household units it uses even when it's perfect. So why is this water use so enormous for plants? Plant physiology is a lot like human physiology. So let's consider breathing. We exhale water vapor to get oxygen. These plants lose water in order to get carbon dioxide. Every square millimeter of the surface of these plants is covered with tiny pores called stomata. The word stomata comes from the Greek word for mouth, so these stomata open to let carbon dioxide in, and they automatically lose water vapor. There's a hundred times more water vapor inside a leaf than there is carbon dioxide in the air and that's why the water use requirement is so enormous. Water has to come out to let the CO2 in. Saving water by closing the stomates is a lot like asking people to save water by stopping breathing. We can't do it. Humans have it easy. There are six hundred times more oxygen in the air than there is carbon dioxide, so that means plants need 600 times more water to grow.

For all the interest in global warming, carbon dioxide is a trace gas, point zero four percent. If we took the air molecules in this auditorium and made them fluorescent, we'd have a hard time finding the carbon dioxide molecules. There is only four carbon dioxide molecules for every 10,000 air molecules. It's one of the great wonders of the world that plants can find those carbon dioxide molecules and make our food, make high-energy food.

To better understand the effect of diet on the environment, let's analyze the land area required to grow the food for one person. So we're joined with this scientist, who has an advanced degree from the Playmobil Institute. And because of our studies with NASA, we've many times analyzed how much land he needs. This green felt represents the land area he needs to grows his own food. It's a small amount of land. If everything's perfect, he grows the food 365 days a year. He can sustain himself on this amount of land. Now we're going to send him into space. After all, we're trying to make a life support system for space. He's got to have some shelter, so we give him a house. But the house covers some of the land. Every photon is precious, so he's got to have a green roof on his house. Now he's ready, growing his own food. But he's going into the vacuum of space. So we're gonna give him a transparent dome, seal it up, recycle every drop of the water, grow the plants at just the right rate so the carbon dioxide and oxygen are in perfect balance, call up Morton Thiokol, put a big rocket under this, off it goes into space. He can go anywhere in the solar system and be self-sustaining, long as he doesn't go too far away from the Sun. What if he gets up one morning and says, “If you please, I would like an egg for breakfast”. He can't do it. We need additional land area to feed this chicken, to give him the egg. What if he says, “I'd like a glass of milk for lunch”? We need even more land area to feed the cow. If he eats the equivalent of 25 percent of his calories from animal products, which is the national average, it more than doubles the land area.

We'll get up each day, my colleagues in animal science, my colleagues in plant science, and work to make water use efficiency in agriculture better, but small changes in our diets can have a much bigger effect than years of our research. Please think about your global food print the next time you think about putting food in the garbage disposal. Please think about that mustard seed and those fossil aquifers, and consider eating less meat. This is the diet for a small planet thank you.

Submitting Your Assignment

Please take the formative assessment quiz in Canvas.

Module 4.2: Impacts of Food Production on Water Resources

Introduction

Agricultural food production impacts water resources by depleting quantities of both surface water and groundwater and by polluting surface and groundwater with pesticides and fertilizers. Module 4.2 includes a brief introduction to impacts of agriculture on water resources, followed by two case studies: Colorado River (flow depletions and salinity) and Mississippi River (nutrients, eutrophication and the hypoxic zone in the Gulf of Mexico).

In completing this module, you will be able to:

  • Attribute major water pollutants to appropriate agricultural sources
  • Summarize the major impacts of agriculture on water resources
  • Relate nutrient loading from fertilizer use to the dead zone in the Gulf of Mexico

Agricultural production has significant impacts on both the quality and quantity of surface and groundwater resources around the globe. In this unit, we'll look at how agricultural activities can contribute to water pollution, and we'll also consider how diversion of irrigation water from both surface and groundwater resources create significant impacts on those water resources and the ecosystems they sustain. Some of the critical issues connecting agricultural activities with water resource quality and quantity are:

  • Agricultural groundwater removal generally exceeds the natural recharge rate, and groundwater overpumping causes irreversible land settling and loss of aquifer storage capacity.
  • Surface water diversion contributes to downstream ecosystem deterioration.
  • Agricultural non-point source pollution is an important contributor to water quality degradation.

Impacts of Water Withdrawals

As discussed in the first part of Module 4, in regions where precipitation is insufficient to grow crops, irrigation water is drawn from lakes, rivers, and aquifers to supplement the insufficient or unreliable precipitation. Water diversions for irrigation can have impacts on both surface and ground water resources.

We saw earlier in this module that the western US receives less precipitation than the eastern US. What does that mean for irrigation needs? The western US withdraws more water from lakes, rivers, and groundwater for irrigation than the eastern US (Figure 4.2.1). These water withdrawals are not without impacts, as well see throughout the rest of this module. Figure 4.2.1 maps the water withdrawal data we explored in the previous unit. Do you remember the three states in the US that diverted the most water for irrigation in the US? California, Idaho, and Colorado. But Nebraska irrigated more acres than both Idaho and Colorado. In the map in Figure 4.2.1, you can clearly see the states that use the most irrigation water. Next, we'll look at some of the impacts of surface and groundwater withdrawals.

Irrigation Water Withdrawals by State 2005
Figure 4.2.1. Irrigation water withdrawals, by State, 2005. The majority of withdrawals (85 percent) and irrigated acres (74 percent) were in the 17 conterminous Western States. The 17 Western States are located in areas where average annual precipitation typically is less than 20 inches and is insufficient to support crops without supplemental water.
Credit: The USGS Water Science School [18]

Impacts of Surface Water Withdrawals for Irrigation

The storage and redistribution of water by dams, diversions, and canals has been a key element in the development of civilizations. Large-scale water control systems, such as on the Nile in Egypt or the Colorado River in the southwestern U.S. make it possible to support large cities and millions of hectares of agricultural land. As the population grows and water diversions increase, serious questions are being raised about the environmental costs of these large water management systems.

Agricultural water withdrawals are placing significant pressure on water resources in water scarce regions around the globe (Figure 4.2.2). If more than 20 percent of a region's renewable water resources are withdrawn, the region is in a state of water scarcity and the water resources of the region are under substantial pressure. If the withdrawal rises to 40 percent or more, then the situation is considered critical and evidence of stress on the functions of ecosystems become apparent. More than 40% of the world's rural population lives in river basins that are physically water scarce and some regions, such as parts of the Middle East, Northern Africa, and Central Asia, are already withdrawing water in excess of critical thresholds (FAO 2011).

In order to divert water from rivers, diversion structures or dams are usually constructed and create both positive and negative effects on the diverted river system. Dams can provide a multitude of benefits beyond their contribution to storage and diversion for agricultural uses. Dams can contribute to flood control, produce hydroelectric power, and create recreational opportunities on reservoirs. Negative impacts of dams and agricultural diversions include:

  • Habitat fragmentation – blocks fish passage
  • Reduction in streamflow downstream, which then results in changes in sediment transport, and in floodplain flooding
  • Changes in water temperature downstream from dam
  • Evaporation losses from reservoirs in hot, dry climates
  • Dislocation of people
  • Sedimentation behind dam fills in reservoirs with sediment and reduces their useful lifespan
Global Distribution of Physical Water Scarcity by Major River Basin
Figure 4.2.2. Global Distribution of Physical Water Scarcity by Major River Basin (FAO 2011)
Credit: © FAO 2011 The State of the World's Land and Water Resources for Food and Agriculture (SOLAW) [19]
Click for a text description of the image

This world map shows that water famine especially high in the Southwestern United States and large areas of Africa, the Middle East, and South Asia.

Impacts of Groundwater Withdrawals for Irrigation

Where surface water supplies are insufficient, groundwater is often used for irrigation (Figure 4.2.3). Agriculture uses about 70% of the groundwater pumped for human use globally and about 53% of the groundwater pumped in the US (USGS: Groundwater use n the United States [20]). In some parts of the world, groundwater is pumped at a faster rate than natural processes recharge the stored underground water. Groundwater use where pumping exceeds recharge is non-renewable and unsustainable.

Another problem that may occur in some aquifers with excessive groundwater pumping is compaction of the aquifer and subsidence of the ground surface. When the water is pumped from the pore spaces in the aquifer, the pore spaces compress. The compression of millions of tiny pore spaces in hundreds of meters of aquifer material manifests on the surface as subsidence. The ground elevation actually decreases. Subsidence from groundwater pumping is irreversible and leaves the aquifer in a condition where it cannot be recharged to previous levels.

Our reliance on and depletion of groundwater resources is becoming a global concern as aquifers are being pumped at unsustainable rates in the US (Figure 4.2.4) and all over the world. Enhanced irrigation efficiencies and conservation measures are being implemented when possible to prolong the life of some aquifers. Unfortunately, groundwater is often the water resource that we turn to in times of drought or when other surface-water resources have been depleted. For example, in California during the recent drought, farmers drilled wells and used groundwater to save their crops when surface water resources were not available.

US map of groundwater withdrawals, by State, 2005
Figure 4.2.3. Groundwater withdrawals, by State, 2005
Credit: USGS: Groundwater use in the United States [21]
Click for a text description of the image

This map of the U.S. shows total groundwater withdraws by state, in millions of gallons per day. California has the highest at 20,000 - 60,000. Nebraska follows at 10,000 - 20,000. Texas and Arkansas are 5,000 - 10,000, Mississippi, Florida, Colorado, Kansas, Arizona, Oregon and Idaho are each 2,000 - 5,000. The rest of the country is 0 - 2000.

Map of the United States (excluding Alaska) showing cumulative groundwater depletion
Figure 4.2.4. Map of the United States (excluding Alaska) showing cumulative groundwater depletion, 1900 through 2008, in 40 assessed aquifer systems or subareas. Colors are hatched in the Dakota aquifer where the aquifer overlaps with other aquifers having different values of depletion.
Credit: USGS: Groundwater depletion [22]

Knowledge Check

Read the following article:

Rosenberg, David M., Patrick McCully, and Catherine M. Pringle. "Global-scale environmental effects of hydrological alterations: introduction. [23]" BioScience 50.9 (2000): 746-751.

Check Your Understanding

Answer the following questions:

What is meant by hydrologic alteration?


Click for answer.

Answer: Hydrologic alteration is an human-made disruption to natural river flows, including dams and diversions. The hydrologic alteration can also include pumping from groundwater, but this article focuses on large dams.

What are the major impacts of hydrologic alteration by dams?


Click for answer.

Answer: Dam can affect both aquatic and riparian ecosystems, block fish passage, change temperature, affect offshore marine areas, contribute to the extinction of species, and affect nutrient cycling. Some rivers, such as the Nile and the Colorado no longer reach the sea.

What is the connection between agricultural food production and hydrologic alteration of the world’s river systems?


Click for answer.

Answer: Globally, 70% of human water consumption is for irrigation, so agriculture is a significant contributor to the impacts of dams and diversions on our river systems.

Water Quality Impacts

Runoff from agricultural areas is often not captured in a pipe and discharged into a waterway; rather it reaches streams in a dispersed manner, often via sub-surface pathways, and is referred to as non-point source pollution. In other words, the pollutants do not discharge into a stream or river from a distinct point, such as from a pipe. Agricultural runoff may pick up chemicals or manure that were applied to the crop, carry away exposed soil and the associated organic matter, and leach materials from the soil, such as salts, nutrients or heavy metals like selenium. The application of irrigation water can make some agricultural pollution problems worse. In addition, runoff from animal feeding operations can also contribute to pollution from agricultural activities.

The critical water quality issues linked to agricultural activities include:

  • Fertilizers – nutrients (nitrogen and phosphorus)
    • Eutrophication – dead zones
  • Pesticides
  • Soil erosion
  • Animal Feeding Operations
    • Organic matter
    • Nutrients
  • Irrigation and return flows
    • Salinity
    • Selenium

Review the following fact sheet on agricultural impacts on water quality:

Protecting Water Quality from Agricultural Runoff, 2005, EPA Fact Sheet on Agricultural Runoff [24]

Check Your Understanding

Answer the following questions:

What is nonpoint source pollution?


Click for answer.

Answer: Nonpoint source pollution derives from diffuse sources, such as agricultural chemicals and fertilizers. The pollutants are picked up by the water as it runs off over the surface or travels through soils and as groundwater, and carried to rivers and lakes.

What agricultural activities contribute to nonpoint source pollution?


Click for answer.

Answer: poorly located or managed animal feeding operation, overgrazing, plowing too often or at the wrong time and improper, excessive, or poorly timed application of pesticides, irrigation water and fertilizers.

What are the major water pollutants contributed by agricultural activities?


Click for answer.

Answer: sediments, nutrients, pathogens, pesticides, metals, and salts

Colorado River Case Study

Flow Depletion and Salinity

The Colorado River in the southwestern U.S. is an excellent case study of a river that is highly utilized for irrigation and agriculture. A majority of the Colorado River’s drainage basin has an arid or semi-arid climate and receives less than 20 inches of rain per year (Figure 4.2.5), and yet the Colorado River provides water for nearly 40 million people (including the cities of Los Angeles, San Diego, Phoenix, Las Vegas, and Denver) and irrigates 2.2 million hectares (5.5 million acres) of farmland, producing 15 percent of U.S. crops and 13 percent of livestock (USBR 2012). Much of the irrigated land is not within the boundaries of the drainage basin, so the water is exported from the basin via canals and tunnels and does not return to the Colorado River (Figure 4.2.6).

The net results of all of these uses of Colorado River water (80 percent of which are agricultural) in both the U.S. and Mexico are that the Colorado River no longer reaches the sea, the delta is a dry mudflat, and the water that flows into Mexico is so salty as a result of agricultural return flows that the U.S. government spends millions of dollars per year to remove salt from the Colorado River.

Many farmers in the Colorado River basin are working to use Colorado River water more efficiently to grow our food and food for the animals that we eat. Watch the video below and answer the questions to learn more about farming in the Colorado River basin.

map shows average annual precipitation of the Colorado River basin
Figure 4.2.5. Average annual precipitation of the Colorado River basin. Data are United States Average Annual Precipitation, 1961-1990 published by Spatial Climate Analysis Service, Oregon State University; USDA - NRCS National Water and Climate Center, Portland, Oregon; USDA - NRCS National Cartography and Geospatial Center, Fort Worth, Texas.
Credit: Map by Gigi Richard
Map of the Colorado River basin showing areas outside of the basin using Colorado River water
Figure 4.2.6. Map of the Colorado River basin showing areas outside of the basin using Colorado River water
Credit: USBR 2012

Check your Understanding

Watch the following video then answer the questions below

Video: Resilient: Soil, water and the new stewards of the American West (10:13)

Resilient: Soil, Water and the New Stewards of the American West
National Young Farmers Coalition [25]

Click for video transcript

Narrator: A drop of water from a sprinkler on a quiet Los Angeles street. A shower head in a Las Vegas hotel. Agricultural land in California's Imperial Valley. Where does all this water come from? The Colorado River. In 1922, representatives from seven states gathered at Bishop’s Lodge New Mexico to sign the Colorado River Compact, an agreement on how to allocate water in this precious river system. But that River had more water then, than it does today. The Colorado River Basin touches the lives of every American. The river system runs through seven states in the US, and two in Mexico, and supplies water for over 36 million people. It also irrigates over five million acres of cropland and provides eighty percent of our winter produce, all from one river. And agriculture is the first to feel the pressure. At the headwaters of the Colorado River, farmers and ranchers are creating a toolbox of resilience. They save water with efficient technology and by building healthy soil.

Brendon Rockey, Rockey Farms, Center, Colorado: My grandpa always had a philosophy on this farm that you have to take care of the soil before the soil can take care of you, and he just felt like we had gotten away from that. That's the number one thing with everybody. is yield, yield, yield. Everybody wants just big production, you know, so that's why you want to dump on the fertilizer, kill off anything that poses a threat. It's all about production. We put more of an emphasis on quality. And what's really nice is when you put the emphasis on quality, the quantity usually comes along with it.

Narrator: And he also uses less water. How? By managing his soil more efficiently and working with nature instead of against it. Brendon rotates his potato crops with green manure, or cover crops, that enhance soil health while reducing his dependence on pesticides, fertilizers, and water.

Narrator: Unhealthy soil lacks life. Often a crust forms on its surface. When a crop is watered, very little soaks into the soil. Instead, it sits on top and is left to evaporate or run off. This land often has to be watered more frequently to get water to the crops. Healthy soils teem with life and are often built when farmers plant a mix of cover crops that add nutrients to the soil. When these plants die they become organic matter which helps store water in the soil. That means farmers can irrigate less, and have more certainty in times of drought.

Brendon Rockey: The reason we got into cover cropping was a response to a drought. Now that we've brought in more diverse crops, that have diverse root systems, which actually help benefit the water use efficiency as well, we've regenerated the soil to the point now where I'm growing a potato crop on about 12 to 14 inches of irrigation water per year. We're focusing on the soil, we're investing in the soil and we're bringing up for the functionality of the soil back to its optimum range.

Mike Jensen, Homegrown Biodynamic Farm, Bayfield, Colorado: A farm after 20 years should have much better soil than when it started. The best thing I do for my land is cover cropping. It rejuvenates the soil keeps everything happy, gets all the flora and fauna in balance. It's not about production this year, it's about production for the next 30 years.

Mike Nolan, Mountain Roots Produce, Mancos, Colorado: One thing I've learned from a bunch of folks, old-timers I've worked with is, do your best to not ever have any bare ground, nothing open, no open soil. I mean even in nature, even in the desert, technically there are things covering the ground. There's things, fungi and bacteria, that are holding the ground together. So what I did about three weeks ago is I planted out this oats crop. I’m not gonna harvest this for the seed or anything, but what it's going to do, it's going to hold moisture in here.

Cynthia Houseweart, Princess Beef Company, Hotchkiss, Colorado: Right now we're in full bloom, but what I like to see is a variety of plants. I don't want to just see straight alfalfa, I want to see grass and, and clover. I don't want bare ground. If we didn't have irrigation water, we would be a desert. This would be sagebrush, cedar trees. This, the water is what creates our livelihood. We graze during the growing season. So the conventional thing is, you move your cows off your pastures, grow them and cut them for hay. What we do instead of cutting them for hay, we graze them.

Narrator: Cynthia waters, using a center pivot. As it moves across her fields, the cattle follow behind eating fresh grass.

Cynthia: And the things they trample in, and their manure, adds to the soil, feeds the soil. It breaks down, turns into humus. The soil becomes more like a sponge and can suck up water that we put on it and rain, so the soil improves, which means the plants grow better and then our cows look better.

Dan James, James Ranch, Durango, Colorado: When you build topsoil, you increase the capillary action of the soils ability to retain water; and the less frequent you're applying your water, the more those roots have to go after that water, as it recedes into the ground. And so now you have all these roots below the surface, and all of a sudden here comes your cow. She comes in and she clips that off. Now your plant’s this high and the plant sheds the same proportion of roots. Now you're adding organic material and you're growing topsoil.

Strengthening the soil is also a concern of Steve Ela, a fruit grower in Hotchkiss Colorado. With precise tools like micro sprinklers and permanent drip irrigation, Steve can use water precisely when and where he needs it most, and his soil is healthy enough to efficiently deliver that water to his crops.

Steve Ela, Ela Family Farms, Hotchkiss, Colorado: For us on the farm it's the difference between using first furrows and the micro specters and now drip. It’s been a bit of an evolution of thinking. So for me it's been, it's not that really one system is better or worse, but it's an evolution of thinking, of trying to manage our water better, trying to use the system of irrigation management and cover cropping to manage our weeds, and also to just only to grow better fruit and healthier trees. Yes, it's expensive on the upfront cost, but it's a system then we can use for 20 years. It's very efficient. I think it probably saves us that much, you know, in water.

Narrator: It's innovation that saves water and money, while increasing soil fertility. It's also innovation that includes technology. Water data delivered by weather satellites, GPS, and even smart sensors like those used by Randy Meaker, a Colorado wheat and corn grower. He uses cover crops to improve his soil and by monitoring soil moisture, he can more effectively use the center pivots to reduce water use.

Randy Meaker: There are huge efforts going on right now, trying to figure out how we and the western United States can solve the shortages of water due to drought conditions. There's two ways to keep water in a bucket and one is to put more water in at the top, the other one is to take less water out at the spigot. People in the lower Basin States, where the population centers are, they're looking for us to supply them more water. But what we're looking for is a responsible use from them. What good is it for me to be restricted if I realize that we're still irrigating lawns, we're still washing cars.

Narrator: Water is the lifeblood of our Western landscape. Farmers and ranchers are as essential to it as the water itself. The water challenges these farmers face are many, but across the country they gather to share their water knowledge and provide each other with valuable support. They build community and grow good food, while stewarding both their land and their water. They are the water stewards of the Colorado River Basin.

Answer the following questions:

How does the Colorado River touch the lives of nearly every American?


Click for answer.

Answer: 80% of winter produce in the US are grown with Colorado River water (in 2014).

What practices are introduced in the film that can increase water use efficiency when growing irrigated crops?


Click for answer.

Answer: Center pivot, Microspray and drip irrigation, Cover crops, Soil health

How can healthy soil reduce the amount of water used to grow crops?


Click for answer.

Answer: Healthy soils can be more permeable and can store more water and so more water soaks into the ground and is stored in the soil. Water in the soil is available to the plants. Increases ability of soil to retain water.

How do cover crops help conserve water?


Click for answer.

Answer: Cover crops help improve soil health (see the previous answer).

Mississippi River Case Study

Dead Zone in the Gulf of Mexico

Agricultural runoff can contribute pollutants to natural waters, such as rivers, lakes, and the ocean, that can have serious ecological and economic impacts, such as the creation of areas with low levels of dissolved oxygen called dead zones caused by pollution from fertilizers. Nutrients, such as nitrogen and phosphorus, are elements that are essential for plant growth and are applied on farmland as fertilizers to increase the productivity of agricultural crops. The runoff of nutrients (nitrogen and phosphorus) from fertilizers and manure applied to farmland contributes to the development of hypoxic zones or dead zones in the receiving waters through the process of eutrophication (Figure 4.2.7).

Schematic of Eutrophication
Figure 4.2.7. Eutrophication
Credit: EPA: Mississippi River/Gulf of Mexico Hypoxia Task Force [26]

Watch the following videos from NOAA’s National Ocean Service that show how dead zones are formed and explain the dead zone in the Gulf of Mexico:

  • Video: Happening Now: Dead Zones in the Gulf 2017 [27] (2:33)
  • Video: Hypoxia [28] (3:51)

The nutrients that make our crops grow better also fertilize phytoplankton in lakes and the ocean. Phytoplankton are microscopic organisms that photosynthesize just like our food crops. With more nitrogen and phosphorus available to them, they grow and multiply. When the phytoplankton dies, decomposers eat them. The decomposers also grow and multiply. As they’re eating all of the abundant phytoplankton, they use up the available oxygen in the water. The lack of oxygen forces mobile organisms to leave the area and kills the organisms that can’t leave and need oxygen. The zone of low oxygen levels is called a hypoxic or dead zone. Streams flowing through watersheds where agriculture is the primary land use exhibit the highest concentrations of nitrogen (Figure 4.2.8).

graph of Nitrogen concentrations in streams draining watersheds with different land uses
Figure 4.2.8. Nitrogen concentrations in streams draining watersheds with different land uses
Credit: Dubrovsky and Hamilton 2010, The Quality of Our Nation’s Waters, Nutrients in the Nation’s Streams and Groundwater: National Findings and Implications [29]

The Mississippi River is the largest river basin in North America (Figure 4.2.9), the third largest in the world, and drains more than 40 percent of the land area of the conterminous U.S., 58 percent of which is very productive farmland (Goolsby and Battaglin, 2000). Nutrient concentrations in the lower Mississippi River have increased markedly since the 1950s along with increased use of nitrogen and phosphorus fertilizers (Figure 4.2.10). When the Mississippi River’s nutrient-laden water reaches the Gulf of Mexico, it fertilizes the marine phytoplankton. These microscopic photosynthesizing organisms reproduce and grow vigorously. When the phytoplankton die, they decompose. The organisms that eat the dead phytoplankton use up much of the oxygen in the Gulf’s water resulting in hypoxic conditions. The resulting region of low oxygen content is referred to as a dead zone or hypoxic zone. The dead zone in the Gulf of Mexico at the mouth of the Mississippi River has grown dramatically and in some years encompasses an area the size of the state of Connecticut (~5,500 square miles) or larger. Hypoxic waters can cause stress and even cause the death of marine organisms, which in turn can affect commercial fishery harvests and the health of ecosystems.

Map of Mississippi and Atchafalaya River Basin and hypoxic zone in Gulf of Mexico
Figure 4.2.9. The Mississippi and Atchafalaya River Basin and the hypoxic zone in the Gulf of Mexico
Credit: USGS Factbook - Nitrogen in the Mississippi Basin-Estimating Sources and Predicting Flux to the Gulf of Mexico [30]
graph of Nitrogen inputs and population from 1940-2010
Figure 4.2.10. Nitrogen inputs and population from 1940-2010
Credit: USGS: Trends in Nutrients and Pesticides in the Nation's Rivers and Streams [31]

Optional Reading

Additional Resources about the Dead Zone in the Gulf of Mexico

  • NOAA sponsored program out of LSU runs the hypoxia net website with great information (Hypoxia In the Gulf of Mexico [32])
  • Hypoxia and Eutrophication from the National Centers for Coastal Ocean Science [33] of the NOAA National Ocean Service
  • USGS Fact Sheet 105-03, 2003, Nutrients in the Upper Mississippi River: Scientific Information to Support Management Decisions [34]

Managing Runoff to Reduce the Dead Zone

What can be done to reduce the size of the dead zone?

The dead zone in the Gulf of Mexico is primarily a result of runoff of nutrients from fertilizers and manure applied to agricultural land in the Mississippi River basin. Runoff from farms carries nutrients with the water as it drains to the Mississippi River, which ultimately flows to the Gulf of Mexico. If a number of nutrients reaching the Gulf of Mexico can be reduced, then the dead zone will begin the shrink.

Since 2008, the Hypoxia Task Force, led by the U.S. Environmental Protection Agency and consisting of five federal agencies and 12 states, has been working to implement policies and regulations with the aim of reducing the size of the dead zone in the Gulf of Mexico. Many of the strategies for reducing nutrient loading target agricultural practices including (USEPA, The [35]Sources [35] and Solutions: Agriculture [35]).

  • Nutrient management: The application of fertilizers can vary in amount, timing, and method with varying impacts on water quality. Better management of nutrient application can reduce nutrient runoff to streams.
  • Cover Crops: Planting of certain grasses, grains or clovers, called cover crops can recycle excess nutrients and reduce soil erosion, keeping nutrients out of surface waterways.
  • Buffers: Planting trees, shrubs, and grass around fields, especially those that border water bodies, can help by absorbing or filtering out nutrients before they reach a water body.
  • Conservation tillage: Reducing how often fields are tilled reduces erosion and soil compaction, builds soil organic matter, and reduces runoff.
  • Managing livestock waste: Keeping animals and their waste out of streams, rivers, and lakes keep nitrogen and phosphorus out of the water and restores stream banks.
  • Drainage water management: Reducing nutrient loadings that drain from agricultural fields helps prevent degradation of the water in local streams and lakes.

Watch the following video from the US Department of Agriculture about strategies to reduce nutrient loading into the Mississippi River:

Video: Preventing Runoff Into The Mississippi River (1:44)

Click for video transcript.
A US Department of Agriculture initiative is helping Missouri farmers keep farm field runoff from reaching the Mississippi River. USDA's Natural Resources Conservation Service is working with producers through the Mississippi River Basin healthy watersheds initiative, or MRBI. The focus of the MRBI project is to hopefully cut down on sediment, nutrients, and pesticides that are moving down the Mississippi River to the Gulf of Mexico. USDA NRCS and initiative partners are working with farmers to determine which conservation practices and which conservation systems work best on their farms to keep runoff from reaching the Mississippi. What we're trying to do is identify what conservation practices will have the biggest impact in the reduction of nutrient transport to the Mississippi River, which eventually makes it to the Gulf of Mexico. One such conservation practice is terracing. By building the terraces it controls erosion, which then reduce the sediment. By reducing sediment we're also going to be reducing the hypoxia issue in the Gulf. We wanted to then monitor what effects we could have on reductions by putting out monitoring stations, such as this one, so we could be able to determine that what benefits we were having with the cost share dollars we were putting on the land. Landowners interested in learning more about how to protect their soil and reduce runoff should contact their local NRCS office. I'm Bob Ellison for the US Department of Agriculture.


EPA website about nutrient pollution and some solutions to nutrient pollution: The Sources and Solutions: Agriculture [36]

Activate Your Learning

Review the graphs below and answer the questions. Figure 4.2.11 presents the size of the hypoxic zone in the Gulf of Mexico from 1985 to 2014. The U.S. Environmental Production Agency led a task force in 2008 that identified a goal to reduce the five-year average of the size of the dead zone to less than 2,000 square miles by 2015.

How close have we come to achieving the action plan goal? How many years between 1985 and 2014 had a hypoxic zone smaller than 2,000 square miles (5,000 square kilometers)?

Click for answer.

Answer: We have not come very close. The 5-year average size of the dead zone from 2010-2014 was about 5,800 square miles and the goal is 2,000 square miles. There was only one year, 2000, between 1985 and 2014 in which the dead zone was smaller than the action plan goal of 2,000 square miles.

Relate the loading of phosphorus and nitrogen to the Gulf of Mexico (Figure 4.2.12 and Figure 4.2.13) to the size of the dead zone. What needs to happen to the annual total nitrogen and phosphorus loading in order to reduce the size of the dead zone?

Click for answer.

Answer: The annual total nitrogen and phosphorus loading to the Gulf of Mexico has remained high from 1980-2011. In order to reduce the size of the dead zone, the annual nutrient loading needs to decrease. There is a goal to reduce the nutrient loading by 45% of the 1980-1996 baseline average.

Propose some possible strategies that could contribute to a reduction in the size of the hypoxic zone. Whose responsibility do you think it is to strategize for reduction of the size of the hypoxic zone? Whose responsibility is it to regulate and enforce strategies for reduction? Who do you think should pay for the proposed strategies? What are some of the challenges associated with your proposed strategies?

Click for answer.

Answer: Answers will vary, but should reflect some of the strategies discussed on the EPA website and video linked above, such as cover cropping, buffers, nutrient management.
Graph of size of the Gulf of Mexico hypoxic zone in mid-summer.
Figure 4.2.11. Size of the Gulf of Mexico hypoxic zone in mid-summer.
Credit: Data source: Nancy N. Rabalais, LUMCON, and R. Eugene Turner, LSU. Funding sources: NOAA Center for Sponsored Coastal Ocean Research and U.S. EPA Gulf of Mexico Program. From Gulf of Mexico Ecosystems & Hypoxia Assessment (NGOMEX) [37].
Graph of Annual Total Nitrogen loads to the Gulf of Mexico
Figure 4.2.12. Annual Total Nitrogen loads to the Gulf of Mexico
Credit: Mississippi River Gulf of Mexico Watershed Nutrient Task Force, 2013. [38]
graph of annual total phosphorus loads to the Gulf of Mexico
Figure 4.2.13. Annual total phosphorus loads to the Gulf of Mexico
Credit: Mississippi River Gulf of Mexico Watershed Nutrient Task Force, 2013. [38]

Summative Assessment

Kansas Farm Case Study

Water is essential to growing food, and the source of water for food production is either naturally occurring precipitation or irrigation from surface or groundwater. The application of fertilizers and pesticides to crops results in the production of water pollution. We can incorporate water resources into our Coupled Human-Natural System diagram, where the climate of the natural system determines the availability of water for food production. The response in the human system is to develop irrigation systems where necessary and implement conservation and efficiency measures in time of scarcity. Also, application of fertilizers and pesticides results in water pollution, which impacts the water quality in the natural system.

schematic of Coupled Human-Natural System

Instructions

In the summative assessment for Module 4, you'll apply what you've learned about coupled human and natural water systems to a particular farming scenario in Pawnee County, Kansas. You'll consider the precipitation in Kansas, the crops you could grow with that precipitation and then look at crop yields for different crops using irrigation. Finally, you'll consider the impact on water resources if you were to shift the types of crops grown and irrigation practices on a farm in Pawnee County, KS. The assignment is explained in the worksheet below.

  • Download Module 4 Summative Assessment Worksheet:
    • Word docx - Module 4 Summative Assessment [39]
    • pdf - Module 4 Summative Assessment [40]
  • Download Excel spreadsheet for calculations for Module 4 Summative Assessment
    • Excel spreadsheet - Module 4 Calculations [41]
  • The discussion portion of the worksheet is incorporated into the weekly discussion post. For the online course, you will not be quizzed on the discussion portion of the assessment but will answer similar questions in the weekly discussion.

Submitting your Assignment

After completing the worksheet, please take the assessment quiz in Canvas.

Grading Information and Rubric (not applicable for online course; grading rubric for in-class assessment activity).

Rubric
Criteria Possible Points
Part 1: Precipitation
Precipitation rates from map are correct 2
Part 2: Crops
List of crops correctly represents crops that could be grown with natural precipitation 4
Part 3: Irrigation Efficiency and Crop Yield
Scenario table correctly populated with results from the Crop Water Allocator 14
Part 4: Discussion and Synthesis
Includes correct usage of the concept of water footprints and connection between diet and water consumption. 5
Clearly explains connections between farming, water scarcity, dead zones, and irrigation efficiency. Also demonstrates clearly the connection between increased water consumption and impacts to water resources, including quality and quantity impacts, such as nutrient pollution and groundwater depletions. 10
Well-written, proper spelling and grammar, and uses complete and well-crafted sentences. 2
Logical presentation of topics. Reasonable length. 3
TOTAL 40

Summary and Final Tasks

Summary

This module has introduced some important concepts that tie our food system to the Earth's water resources. Water resources are essential for food production, and food production also has significant impacts globally on both the quantity surface and groundwater and the quality. Growing crops relies on water from either precipitation or irrigation derived from surface and groundwater. Virtual water is embedded in everything you eat, with the amount of water varying, depending on the crop and the climate in which the crop was grown. Crops grown in hot and dry climates consume more water via transpiration as evaporation rates are higher in those climates. Also, some plants need more water than others, for example, rice uses more water to grow than corn. You explored precipitation rates in different parts of the US compared to evaporation rates and considered how much water might need to be applied to certain crops. Computation of your personal water footprint allowed you to compare your lifestyle and resulting water consumption with average water consumption in the US and globally. Also, these computations along with consideration of virtual water in different food products allowed you to draw conclusions about the impacts of different types of diets on the planet's water resources.

In this unit, we've just touched the surface of the very large issue of how agriculture impacts both the quality and quantity of our water resources. We also looked at a few examples of agricultural practices that help to minimize and reduce these impacts.The Colorado River provided an example of a river on which agricultural diversions have severely impacted the quantity of water in the river. We saw that the Colorado River no longer reaches the sea! The breadbasket of the US, the Midwest, contributes nutrient pollution to the Mississippi River, which has, in turn, created a massive dead zone in the Gulf of Mexico. You explored data on the size of the dead zone and proposed strategies to reduce the nutrient loading and thereby reduce the size of the dead zone in the future.

Reminder - Complete all of the Module 4 tasks!

You have reached the end of Module 4! Double-check the to-do list on the Module 4 Roadmap [42] to make sure you have completed all of the activities listed there before moving on to Module 5!

References and Further Reading

  • Dubrovsky, N.M. and P.A. Hamilton, 2010, Nutrients in the Nation’s Streams and Groundwater: National Findings and Implications, USGS Fact Sheet 2010-3078 (http://pubs.usgs.gov/fs/2010/3078/ [43])
  • FAO, 2011, The State of the World’s Land and Water Resources for Food and Agriculture (SOLAW) - Managing systems at risk. Food and Agricultural Organization of the United Nations, Rome and Earthscan, London.
  • Goolsby, D.A., and Battaglin, W.A., 2000, Nitrogen in the Mississippi Basin--Estimating sources and predicting flux to the Gulf of Mexico: U.S. Geological Survey Fact Sheet 135-00, 6 p.
  • Hoekstra, A.Y. and M.M. Mekonnen, 2012. The water footprint of humanity, Proceedings of the National Academy of Science, vol. 109, no. 9, pp. 3232-3237 (http://waterfootprint.org/media/downloads/Hoekstra-Mekonnen-2012-WaterFo... [13]).
  • Jones, J.A.A., 2010, Water Sustainability: A Global Perspective, Hodder Education, 452 pp.
  • Mississippi River Gulf of Mexico Watershed Nutrient Task Force, 2013, Reassessment 2013: Assessing Progress Made Since 2008, Accessed from http://www2.epa.gov/sites/production/files/2015-03/documents/hypoxia_rea... [44]
  • U.S. Bureau of Reclamation (USBR), 2012, Colorado River Basin Water Supply and Demand Study, http://www.usbr.gov/lc/region/programs/crbstudy/finalreport/index.html [45]

Additional Resources

  • Global annual precipitation map [46]
  • FAQ - CROP WATER USE [47], Agricultural Water Conservation Clearinghouse
  • Good Virtual Water resource from the Water Footprint Network [48]
    Product gallery with virtual water for different agricultural products and citations for all of their data and maps

Videos

  • Invisible water, the hidden virtual water market [49]| Seth Darling | TEDxNaperville
  • Virtual Water [50]
    Video introducing virtual water in liters, used on British news to mark World Water Day 2007 and broadcast on Five and Sky News in the UK)
  • ALL YOU EAT,  [51]UN Water, World Water Day
    Short graphical video showing the liters of virtual water in a few food products
  • Water 101 - #1 Water for Food [52], produced by FAOWATER [53]
    Short animation introducing the relationship between food production and water use

Module 5: Soils as a Key Resource for Food Systems

Overview

Interactions Between Soil Nutrients, Nutrient Cycling, and Food Production Systems

Along with water, sunlight, and earth's atmosphere, the soil is one of the key resources underlying food production by humans. In terms of the coupled human-natural systems we use as a way to understand food systems, we can say that human systems organize landscapes and manage soils, along with agricultural biodiversity and other parts of natural systems, to produce food. Soils exert an influence on this coupled system because they vary in terms of properties such as depth and nutrient content, which alters their response to human management. Soils also have great importance as the site of many nutrient and carbon transformations within the biosphere. They are a storehouse of beneficial soil organic matter that benefits the earth system in many ways. Also, by understanding soils and the earth surface and ecological processes that occur there, human management is able to maintain and improve them, as well as overcome initial limitations or past degradation.

The purpose of this module is to give you as a learner a basic grounding in the nature of soils and soil nutrients. Module 5.1 provides the foundation for understanding soils, soil nutrients and their connection to food. We will also focus on ways that soils are vulnerable to degradation that impairs their role in food production. In module 5.2 we will deepen understanding of how soil management can protect soils in their role of supplying nutrients to crops and protecting other valuable resources such as surface water. To accomplish this we will focus on nitrogen (N) and phosphorus (P) as key nutrients for food production in module 5.2.

Goals and Learning Objectives

Goals

  • Identify soil nutrients and soil function as key resource in need of protection for food production and food systems.
  • Describe spatial and geographic variation in soil resources and soil fertility.
  • Distinguish between preexisting aspects of biogeochemical cycling and human-induced processes that affect biogeochemical cycling.
  • Attribute different soil fertility outcomes in food systems to the coupled natural and human factors and feedbacks that produce them.

Learning Objectives

After completing this module, students will be able to:

  • Describe the basic properties of soil that distinguish it from mere "dirt".
  • Explain how soil serves as a medium for plant growth.
  • Explain how the five soil-forming factors interact to produce soils.
  • Explain the term "biogeochemical cycling".
  • Explain common limiting factors to plant growth that limits food production around the world.
  • Explain how nutrient and carbon depletion from soils and soil erosion create conditions of low food productivity.
  • Assess how farming practices affect soil fertility.
  • Analyze modern fertilizer use as the emergence of a strong human system impact on nutrients in soils that replenishes soil nutrients but can create nutrient pollution.
  • Analyze how natural/human system feedbacks operate to limit the actions of poorer food producers around the world.
  • Incorporate sustainability challenges related to soil nutrient management into an analysis of food systems.

Assignments

Module 5 Roadmap

Detailed instructions for completing the Summative Assessment will be provided in each module.

Assignment Location
Module 5 Roadmap
To Read
  1. Materials on the course website.
  2. Chapter 2, pp. 9-17 in Building Soils for Better Crops [54] (USDA Sustainable Agriculture Research and Education), available as a free e-book [55]. You can download the entire book since future modules will also use this source.
  1. You are on the course website now.
  2. Building Soils for Better Crops [56]
To Do
  1. Formative Assessment: Mapping Trends in Soil Properties
  2. Summative Assessment: N and P Balances
  3. Participate in the Discussion
  4. Take Module Quiz
  1. In course content: Formative Assessment [57]; then take quiz in Canvas
  2. In course content: Summative Assessment [58]; then take quiz in Canvas
  3. In Canvas
  4. In Canvas

Questions?

If you prefer to use email:

If you have any questions, please send them through Canvas e-mail. We will check daily to respond. If your question is one that is relevant to the entire class, we may respond to the entire class rather than individually.

If you prefer to use the discussion forums:

If you have any questions, please post them to the discussion forum in Canvas. We will check that discussion forum daily to respond. While you are there, feel free to post your own responses if you, too, are able to help out a classmate.

Module 5.1: Soil basics

Introduction

Overview of soils and nutrients for food production

In this course, we describe food systems as a coupling between human societies and natural earth systems and environments. This coupling is especially clear in the activities of food production that rely on crop and livestock raising. Crops and livestock production (and to a similar extent, fisheries, and aquaculture) require food producers bring together human management with soil conditions and soil nutrients (this module), water (next module), as well as sunlight for energy and adequate climate conditions (temperature, humidity, adequate growing season). To understand these human-natural interactions across the entire course, and to build your capacity to understand natural factors as part of your capstone projects and other chapters of your education, this module describes basic soil properties and the role of soils in creating adequate conditions for crops to grow, which underlies most aspects of food production. It’s therefore very important that we understand soils as the “living skin of the earth” in their properties and history, the global patterns of soil fertility and soil limitations, and then its role in supplying nutrients to plants, and how soil fertility is regenerated by the human societies and management knowledge that allows them to continue supporting food production. Our goal is not to condense an entire course in soil science, although we hope that many of you will go on to take such a course. Rather, we want to sketch out major factors and determinants of the opportunities and limitations posed by soils to a human food production system.

What is Soil?

We may be used to referring to soil as “dirt”, as in “my keys fell in the dirt somewhere” or “after planting the garden we had dirt all over our hands” but the way in which soil supports food production far more complex than a smear of clay on our hands. One way to define this difference in perspective is to think about the biological and chemical complexity in soil, and the fact that soils are not just brown, powdery handfuls of dirt but occupy a grand scale in the natural systems that underlie food systems. Soil is the "skin of the earth", layers that ascend from bedrock and supply water and nutrients to the fields and forests that make up the terrestrial biosphere. Soils are ecosystems in their own right, within mineral layers that form part of earth’s surface. Soils can be as shallow as ten centimeters and as deep as many tens of meters.

An interesting exercise is to think of a single term or concept that describes how soils work and what they are. For example, if we were seeking an acronym to describe soil and market it as the marvelous thing that it is1 —and if we lacked time to think of a catchier name – we might think up the acronym “PaBAMOM” which nevertheless is a pretty good summary of what soil is: a “Porous and Biologically Active Mineral-Organic Matrix”. It’s a good summary because it defines the unique properties of soils (see Figure 5.1.1 below):

  1. Porous (full of open spaces or pores), at a range of pore sizes from well below a micron (10-6 m or 0.0001 cm) to many centimeters, and therefore able to store water and transmit it to deeper earth layers, and host organisms as diverse as bacteria, plant roots, and prairie dogs. This porosity arises not only from the inherent sizes of particles in soil but is also a result of soil organisms and roots that generate aggregation of the soil from clay and silt particles into crumbs and clods that may be familiar to from a typical garden soil. This biologically generated aggregation is sometimes referred to as structure, which is seen in figure 5.1.1 as the overall arrangement of pores and aggregates, and in the notion that soil is a matrix (point five below). The ideas of aggregation and structure will be revisited in this module and in module 7.
  2. Soil is phenomenally biologically active and biologically diverse, especially in microbes, which makes it able to perform many useful functions. For example, soil microbes are able to "recycle" or decompose materials like wood, wheat straw, and bean roots into energy for themselves and other soil biotas; various other types of microbes also draw or fix nitrogen out of the air to feed plants, detoxify the soil from organic pollutants, or perform myriad other beneficial services -- and other not so beneficial processes such as diseases.
  3. Soil is mineral, formed from the breakdown and chemical processing of the earth’s rock crust into sand, silt, and clay, each with their own ability to store water based on the size of pores they create and unique chemical roles in further processing and breakdown of soil materials. Usually, the mineral part is most of the solid (non-pore) part of soil (Figure 5.1, top pie chart)
  4. Soil is also organic, containing bits of organic (carbon-containing) remnants of plants and animals, some of which become stabilized until they last hundreds and even thousands of years as part of the soil. In the current efforts to promote carbon sequestration to alleviate (mitigate) climate change, it is worth noting that the amount of carbon stored in these bits of soil organic matter globally easily exceeds the total carbon stock in all of the planet’s forests.
  5. Lastly, it is a matrix, which means that at least as important as the particles, aggregates and pores of the soil are the organisms and processes that occur on and in these particles and pores (Fig. 3.1, bottom). This matrix hosts a highly complex ecosystem that winds its way through the millions of pores, roots, fungal hyphae, insects, and other organisms in soil. And here we are referring to the complex system concept we presented in module one: soil has many interacting parts with overlapping interactions, the ability to produce unexpectedly stable or unstable outcomes, and contains processes that can produce positive and negative feedbacks. One important example of this type of behavior is the range of soil productivity “behavior” of soils over time, including the ability of some soils to sustain moderate to high levels of productivity over years or decades (they resist change through processes of negative feedback), and then collapse in terms of food production as the interlocking, complex systems fueled by organic constituents and biological processes are dismantled (positive feedbacks operate to drive them towards degradation). Soils can then be similarly resilient in terms of remaining unproductive until the complex systems can be rebuilt through soil restoration practices.
Pie Chart, see description above for more details.Diagram, see description above for more details
Fig. 5.1.1. Top: pie chart showing the typical physical composition of most soils used in food production; Bottom: Basic cross-section of approximately 20 mm wide of soil as a porous, biologically active mineral-organic matrix. Red arrows show non-living components while purple arrows show biological components. Large macropores resulting from good soil structure allow adequate drainage and air entry to a soil for biological activity, while smaller mesopores and micropores hold water at varying degree of availability for plant roots. Macrofauna such as earthworms (>2mm approximate dimension) are also very important but would occupy too much of the diagram to show.
Credit: Steven Vanek, adapted from Steven Fonte.

So, soil is not dirt. It is porous and complex, it covers almost every land surface on the planet (ice caps, glaciers, and bare rock are exceptions), and it is a ubiquitous, critical resource that is heavily coupled to human societies for their food production and in need of protection. It’s not dirt, it’s a PaBAMOM!


1. We don’t have to do this marketing job (phew!) because the existence and value of soils are so often taken for granted. Recently, economists have been working on estimating the implicit worth of the services performed for society by a single hectare (100 m x 100 m) of soil, and the amounts can range into tens of thousands of dollars per year depending on soils’ properties and the way they are used.

Soils Support Plant Growth and Food Production

Support, Water, and Nutrients

Before examining other basic soils functions, it is helpful and will avoid possible confusion, to understand the basics of how soils support the needs of crops, which in turn support the food needs of humans and their livestock. Firstly, soils provide a physical means of support and attachment for crops – analogous to the foundation of a house. Second, most water used by plants is drawn up through roots from the pores in soils that provide vital buffering of the water supply that arrives at crops either from rainstorms or applied as irrigation by humans. Third, as crops grow and build their many parts by photosynthesizing carbon out of the air (see module 6, next, for more on this) they gain most of the mineral nutrients they need (chemical elements) they need2 from soils, for example by taking up potassium or calcium that started out as part of primary minerals in earth’s crust, or nitrogen in organic matter that came originally from fertilizer or the earth’s atmosphere. The adaptation of crop plants domesticated by human farmers (and other plants) to soils, and the adaptation of the soil ecosystem to plants as their primary source of food mean that soils usually fulfill these roles admirably well.


2 The elements needed by plants other than Carbon (from the air) and Hydrogen/Oxygen (from water) in rough order of concentration are Potassium, Nitrogen, Phosphorus, Calcium, Magnesium, Sulfur, Iron, Manganese, Zinc, Boron, Copper, Molybdenum, and Cobalt (for some plants). Other elements are taken up into plants in a passive way without being essential, such as Sodium, Silicon, or Arsenic.

Soil Formation and Geography:

How do soils form in different places?

Soil Formation Factors

Soils around the world have different properties that affect their ability to supply nutrients and water to support food production, and these differences result from different factors that vary from place to place. For example the age of a soil -- the time over which rainfall, plants, and microbes have been able to alter rocks in the earth's crust via weathering-- varies greatly, from just a few years where soil has been recently deposited by glaciers or rivers, to millions of years in the Amazon or Congo River Basins. A soil's age plus the type of rock it is made from gives it different properties as a key resource for food systems. Knowing some basics of soil formation helps us to understand the soil resources that farmers use when they engage in food production. Below are some of the most important factors that contribute to creating a soil:

  1. Climate: climate has a big influence on soils over the long term because water from rain and warm temperatures will promote weathering, which is the dissolution of rock particles and liberating of nutrients that proceed in soils with the help of plant roots and microbes. Weathering requires rainfall and is initially a positive process that replenishes these solubilized nutrients in soils year after year and helps plants to access nutrients. However over the long run (thousands to millions of years) and in rainy climates, rain water passing through a soil (leaching) leaves acid-producing elements in the soil like aluminum and hydrogen ions, and carries away more of the nutrients that foster a neutral pH (e.g. calcium, magnesium, potassium; see the next page on soil properties for a discussion on soil pH). Old soils in rainy areas, therefore, tend to be more acidic, while dry-region soils tend to be neutral or alkaline in pH. Acid soils can make it difficult for many crops to grow. Meanwhile, dry climate soils retain nutrients gained in weathering of rock -- a good thing -- but may lack plant cover because of dry conditions. A lack of plant cover leaves the soil unprotected from damage by soil erosion and means that dry climate soils often lack dead plant material (residues) to enrich the soil with organic matter. Both dry and wet climate soils have advantages as well as challenges that must be addressed by human knowledge in managing them well so that they are protected as valuable resources.
  2. Parent material: soils form through gradual modification of an original raw material like rock, ash, or river sediments. The nature of this raw material is very important. Granite rock (magma that hardened under the earth) versus shale (old, compressed seabed sediments) produce very different soils. An important example of parent material influencing soils with consequences for human food production are soils made from limestone or calcium and magnesium carbonates. These rocks strongly resist the process of acidification by rainfall and leaching described above. Limestone soils maintain their neutral level of acidity (or pH) even after thousands of years of weathering, and thus can better maintain their productivity. An example of this parent material influence is the Great Valley in Pennsylvania, USA, where the Amish reside. These Pennsylvania soils are considered some of the most productive soils in the U.S. even after hundreds of years of farming. Pockets of other limestone soils the world over are similarly productive over the long term. In summary, as part of learning about a food production systems of a region, it can be helpful to consider the types of rock that occur in that region, which you may want to consider for your capstone regions.
  3. Soil age: the time that a soil has been exposed to weathering processes from climate, and the time over which vegetation has been able to contribute dead organic material, are important influences on a soil. Very young soils are often shallow and have little organic matter. In a rainy climate, young (e.g. 1000 years) to medium aged (e.g. 100,000 years) soils may be inherently very fertile because rainfall and weathering have not yet removed their nutrients. Old soils are usually deep and may be fertile or infertile depending on the parent material and long-term climatic conditions. Soils in previously glaciated regions such as the northern U.S. and Europe are usually thought of as young because glaciers recently (~10,000 years ago) left fresh sediments made from ground up rock materials.
  4. Soil slopes, relief, and soil depth: Steep slopes in mountain and hilly regions causes soils to be eroded quickly by rainfall unless soils are covered by throughout the year by crops or forest. These hilly and mountain regions may also have young soils, and the combination of young soils and erosion can make for soils that are quite thin. Meanwhile, flat valley areas are where the eroded soil is likely to accumulate, so soils will be deep. Along with the water holding capacity and the nutrient content of a soil, soil depth determines how much soil "space" or soil volume a crop's roots can explore for nutrients and water. Soil depth is an important and often overlooked determinant of crop productivity of soils. Moreover, these large-scale "mountain versus valley" differences can be mirrored within a single field, with small differences in topography creating differences in drainage, depth, and other soil properties that dramatically affect soil productivity within ten to twenty meters distance.

A Summary of Soil Formation: The Global Soils Map

These four factors along with the vegetation, microbes, and animals at a site, create different types of soils the world over. A basic global mapping of these soil types is given below in Fig. 5.1.2 We've attached some soil taxonomic names (for soil orders, categories used by soil taxonomists) to these basic soil types for those who are familiar with some of the terminology of soil classification. We should emphasize that understanding these orders is not essential to your understanding of food production and food systems, as long as you understand how the basic processes of soil formation described above, and the properties of soils described on the next page, contribute to the overall productivity of a soil. You should think about how the soil formation processes affect crop production in your capstone regions of your final project, and you should be able to find resources on how soils were formed in any place in the United States and around the world.

Simplified global soil map classified into broad categories.
Figure 5.1.2. Simplified global soil map classified into broad categories.
Credit: Steven Vanek based on USDA world soils map.

Formation and Management Affect a Soil's Productivity

Another important point is that soil formation processes described above largely determine only the initial state of a soil as this passes into human management as part of a coupled human-natural food system. Human management can have equally large effects as soil formation on productivity, either upgrading productivity or destroying it. The best management protects the soil from erosion, replenishes its nutrients and organic matter, and in some ways continues the process of soil formation in a positive way. &We'll describe these best practices as part of a systems approach to soil management in module 7. Inadequate human management can be said to "mine" the soil, only subtracting and never re-adding nutrients, and allowing rainfall and wind to carry away layers of topsoil.

The next page adds to this description of soil formation by focusing in on the basic properties that affect food production on soils, like acidity and pH which is discussed above.

Check Your Understanding: The Nature of Soil and Soil Formation

Answer the following questions on basic properties of soil and factors of soil formation:

The two basic types of chemical nature of materials in the solid part of soils (i.e. not air and not water) are mineral and __________.


Click for answer.

ANSWER: organic

2. Fill in the blanks using the choices given: Large pores in soil are important for _______________ and medium size pores are important for _____________________. (choices: holding water for plants, drainage)


Click for answer.

ANSWER: first blank: drainage; second blank: holding water for plants

The amount of carbon held in soils globally is (larger, smaller) than the amount of carbon in all the earth's forests.


Click for answer.

ANSWER: larger

Soils with what type of rock tend to resist the natural process of acidification that happens to soils in many climates?


Click for answer.

ANSWER: limestone

In global comparative terms and considering the global soil map above, a soil in the Northeast United States would tend to be ____________________ (young or old).


Click for answer.

ANSWER: These soils are relatively young. An additional note: this is especially true because of recent (100,000 to 10,000 years B.P.) glaciation and exposure of new ground rock and silt by glaciers that created the conditions for the formation of these young soils all through the temperate regions.

Which of the following is likely to have a low level of organic matter and why? A dry climate soil or a wet climate soil?


Click for answer.

ANSWER: the dry climate soil, because plant growth is limited and there is less dead plant material going into the soil

Soil Properties and Human Responses to Boost Food Production

Nutrients, pH, Soil Water, Erosion, and Salinization

In growing crops for food, farmers around the world deal with local soil properties that we started to describe on the previous page. These properties can either be a positive resource for crop production or limitations that are confronted using management methods carried out by farmers. The first of these, a soil's nutrient status, is described in more detail in module 5.2. Regarding nutrients is only important to emphasize here that most nutrients taken up by plants (other than CO2 gas) come to plant roots from the soil, and that the supply of these nutrients often has to do with the amount of dead plant remains, manure, or other organic matter that is returned to the soil by farmers, as well as fertilizers that are put into soils to directly boost crop growth. Here are the other major soil properties that farmers pay attention to in order to sustain the production of food and forage crops:

Soil pH or Acidity: Near Neutral is Best

Most crops prefer soils that have a pH between 5 and 8, mildly acidic to mildly alkaline (to understand these pH figures, remember that water solutions can be either acidic or basic (alkaline), and that pH 7 is neutral, vinegar has a pH of about 2.5, and baking soda in water creates a pH of about 8). As discussed above under the climate and parent material sections describing soil formation, soils in rainy regions tend to become more acidic over time.& Soils with too low a pH will have trouble growing abundant food or feed for animals. Farmers manage soils with low pH by adding ground up limestone (lime) and other basic (that is, acid-neutralizing) materials like wood ash to their soils. As an alternative, farmers sometimes adapt to soil pH by choosing or even creating crops or crop varieties that have adapted to low pH, acidic soils. For example, potatoes do well in high elevation, acidic soils of the Andes and other areas around the world. Alfalfa for livestock does better in neutral and alkaline soils while clovers for animal food grow better in more acidic soils.

Soil Water Holding Capacity and Drainage: Deep, Loamy, and Loose is Best

Module 4 described the importance of water for food production and the way that humans go to great lengths to provide irrigation water to crops in some regions. Soil properties also play a role in the amount of water that can be stored in soils (for days to weeks) that is then available to crops. A soil that holds more water for crops is more valuable to a farmer compared to a soil that runs out of water quickly. Among the properties that create water storage in soils is soil depth or thickness, where a deep soil is basically a larger water tank for plant roots to access than a thin soil. The proportions of fine particles (clay) versus coarse particles (sand) in a soil, called soil texture, also influences the water available to plants: Neither pure clay nor pure sand hold much plant-available water because clay holds the water too tightly in very small pores (less than 1 micron or 0.001 mm, or smaller than most bacteria) while sand drains too rapidly because of its large pores and leaves very little water. Therefore an even mix of sand, clay, and medium-sized silt particles holds the maximum amount of plant available water. This soil type is known as loamy, which for many farmers is synonymous with “productive”. In addition to these soil properties, farmers try to maintain good soil structure (also called "tilth"), which is the aggregation of soil particles into crumb-like structures, that help to further increase the ability of soils to retain water. Soil aggregation or structure, and its multiple benefits for food production are further described in Module 7 on soil quality.

Clayey soils, and soils that have been compacted by livestock or farm machinery ("tight" vs. "loose" soils), can also have problems allowing enough water to drain through them (poor drainage), which can lead to an oversupply of water and a shortage of air in soil pores (refer back to figure 5.1.1 and the roughly equal proportion of air and water in pores of an agricultural soil). Too much water and too little air in a soil lead to low oxygen in the soil and an inability for roots and soil microbes to function in providing nutrients and water to plants. Part of good tilth, described above, is maintaining a loose structure of the soil.

In the face of these important soil properties for water storage, farmers seek out appropriate soils with sufficient moisture (e.g. deep and loamy, see Figs. 5.1.3 and 5.1.4) but also adequate drainage. Food producers also modify and maintain the moisture conditions of soils, through irrigation but also through maintaining good soil aggregation or tilth (see modules 5.2 and 7), and by avoiding compaction of soils that also leads to poor drainage and soils that are effectively shallower because roots cannot reach down through compacted soils to reach deeper water.

Shallow soil
Fig. 5.1.3. The shallow soil with an oat crop is in a mountainous region that has likely suffered erosion, and features of the bedrock can be seen within 50 cm of the surface (yellow line), where the soil becomes much poorer. The total volume available to store nutrients and water in this soil is low. A pick axe head is shown for scale.
Credit: Steven Vanek
deep soil
Fig. 5.1.4. This loamy, deep soil is likely in a flatter region and has an organic-matter rich layer that extends to about 40 cm below the surface and water storage capability to beyond one-meter depth (numbers on tape are cm), an excellent nutrient and water resource for food production.
Credit: Stan Buol, North Carolina State University Soil Science, on Flickr [59] [59] Creative Commons (CC BY 2.0)

Salinization and Dry Climates: Hold the salt

Dry climate soils have less rainfall to leach them of minerals. They can, therefore, be high in nutrients, but also carry risks of harmful salts building up as rainfall does not carry these away either. Salt-affected soils may either be too salty to farm at all or may carry a risk that if irrigation water is too high in salts or applied in insufficient amounts to continually “re-rinse” the soil of salts, then salts can build up in soils until crops will not grow. The way that arid soils are managed is a key part of the human knowledge of food production in dry regions.

Relief and Erosion: Don't Let Soil Wash Down the Hill

Soil slope and relief are described on the previous page as creating higher risks of erosion (Fig. 5.1.5). To address this limitation food producers have either (a) not farmed vulnerable sloped land with annual crops, leaving them in forest, tree crops, and year-round grass cover and other vegetation that holds soils on slopes; (b) built terraces and patterned their crops and field divisions along the contours of fields (Fig. 5.1.6). Terracing and terraced landscapes can be seen from Peru to Southeast Asia to Greece and Rwanda. Nevertheless, while sloped soils have been seen as the Achilles heel of environmental sustainability in mountain areas, the extreme elevation differences present in mountain areas can also be seen as a benefit to these food systems. The benefits arise because soils with very different elevation-determined climates and soil properties in close proximity, which allows for the production of a greater variety of crops. The simultaneous production in the same communities of cold- and acid soil tolerant bitter potatoes and heat-loving maize and sugar cane in lower, more neutral soils in the Peruvian Andes is an example of this benefit in high-relief mountain regions.

Soil erosion
Fig. 5.1.5. Soil erosion in a mountain landscape
Credit: Steven Vanek
terrace irrigation
Fig. 5.1.6. Terracing in a mountain landscape.
Credit: Quinn Comendent, used with permission under a creative commons license.

Soil Health: Understanding Soils as an Integrated Whole for Food Production

We hope that you are beginning to appreciate that appropriate management of soils is emphatically about integrating management principles like the ones presented here as human responses, along with an understanding of the basic properties of soils, and also the nutrient flows presented next in module 5.2. Soils are very much a complex system, and managing them for food production and environmental sustainability means that we must understand the multiple components and interactions of this system. The way in which this is accomplished has been summarized as the concept of Soil Health, which involves multiple components that are more fully addressed in module 7. Soil health is an aspiration of effective management and means that management has maintained or promoted properties like nutrient availability, beneficial physical structure, and a diversity of functionally important and 'health-promoting' microbes and fauna in soils along with sufficient organic matter to feed the soil ecosystem. These integrated properties then allow production to avoid soil degradation, produce sufficient amount of food and livelihoods, and preserve biodiversity in soils as well as other significant ecosystem services like buffering of river flows and storage of carbon from the atmosphere.


3 This is not always true; Molybdenum, Sulfur, Boron and other micronutrients are sometimes found to limit plants, but the complexity of analyzing these is beyond the scope of this survey course.

Formative Assessment

Mapping Trends in Soil Properties

Instructions

You will complete an activity on mapping trends in soil properties using an online soil mapping resource. The emergence of tools such as this to visualize global and national soil data easily and with full public access is revolutionizing information about soils and management constraints in different regions of the world. Please download the worksheet so that you can fill it in (either on paper or preferably just by writing in your responses in MS Word).

The two web resources you will need for this worksheet are placed here so you can access them while you fill in the worksheet.

Mainly you will need the International Soil Resource Information Centre's soil mapping resource of the world, Soil [60]Grids [60]. Click past the intro window that will appear in the center of the screen and then pan the map to the area of interest as identified in the worksheet.

example of a map from the SoilGrids data portal
Fig. 5.1.7. An example of a map from the SoilGrids data portal. The layer that is shown is the global map of clay content (% clay) in soils, where areas that have more purple are higher in clay content in their surface soils.
Credit: image above and in the downloadable worksheet were generated using the SoilGrids data portal, and are used with permission of the International Soil Reference and Information Centre (ISRIC) according to the terms of an open database license (ODbL). Sharing and adapting of data is permitted.

This is a mapping portal that resembles google earth - you have the ability to pan, zoom in, drag the map with the cursor and mouse (Fig. 5.1.7). When you enter you should see a toolbar in the top right corner. More instructions on the portal are given on the formative assessment worksheet.

You will also need briefly, this online map showing global annual total precipitation [61].

Files to Download

Download the Worksheet [62]to complete your assessment.

Submitting your assignment

For on-line course after completing the worksheet, please take the assessment quiz in Canvas. For hybrid course please submit your assignment in Canvas.

Grading Information and Rubric (not applicable for online course; grading rubric for in-class assessment activity).

Your assignment will be evaluated based on the following rubric. The maximum grade for the assignment is 30 points.

Rubric
Work Shown Possible Points
Understanding and correctly querying data on the web resource 10
Filling in both columns of table for pH 3
Seven response areas in questions 9 through 15, 2 points each for
partial credit and complete credit based on correctness and completeness
14
Style and grammar elements within written responses that require sentences 3

Understanding Soil Maps at a Broad Global Level

Soil scientists have done an enormous amount of work in mapping the patterns of soil at a global level. The most current and detailed effort comes out of mapping work from the Food and Agriculture Organization of the United Nations, now an independent agency that is known as the International Soil Resource Information Centre (ISRIC), and is based on classifying a set of diagnostic types of topsoil layers that occur in different climates, landscape ages, and vegetation types. The details of this system5 are beyond the scope of this course, however, and to summarize the introduction to global soil fertility in this unit we present a simplified version of the United States Department of Agriculture (USDA) system that is still in wide use by soils practitioners in the United States. The USDA system lines up very well with the ISRIC system at this simplified level and allows understanding of the broad strokes of soil nutrient geography in the way we have presented it (Figure 3.8).

Soil types in different parts of the world. See more details below.
Fig. 3.8. Simplified map of soil types in the world and associated characteristics, referred to the USDA soil classification system.
Source: adapted by S. Vanek from the USDA Natural Resource Conservation Service (NRCS) [63]

This simplified map is intended to serve as a resource for your other learning in the course on how food systems may respond to the opportunities and limitations of soils, and also summarizes the learning in this module about how soils result from an interaction of parent material, time, climate, vegetation, and other factors. For example, you’ll notice that just four very broad summarized types (See section 1 of the soils key, “Dominant global soils” in Fig. 3.8) cover the vast majority of the earth’s surface, and can be organized into a rough typology of precipitation from wet to dry, along with their age and vegetation types (e.g. tropical and subtropical forests; other forest types; grasslands, and desert vegetation). Soils formed by temperate grasslands have been hugely important in recent history because once humans developed steel plows that were sufficiently strong to til prairie soils, these Mollisols could be farmed and became the breadbaskets of the modern era (e.g. the U.S. and Canadian Great Plains, the Ukraine, the Argentinian pampas). There are also small pockets of soils globally that depend strongly on their original parent material. Andisols or volcanic ash soils are an excellent example of this: although their global extent is minuscule and even invisible on our map (Fig. 3.8) at this scale, they often occur in areas with high population densities such as Ecuador, Japan, and Rwanda. The high densities of population are not an accident but occur exactly because these soils have high fertility potential and have become extremely important in these local food systems. The simplified global soils map is also a way to spatially conceptualize a number of key limiting factors in soils that food producers must face: acidic, P-retaining soils in highly weathered tropical and subtropical soils, P retention in volcanic soils, and the risk of salinization of soil in dry climate soils.

In addition, it is worth noting that the broad swaths of soil of young to moderate age and with moderate to high fertility (light green in our map) may be the dominant type of soil in the world and also includes many areas that are critical in terms of the sustainability outcomes for human-natural systems in relation to soils. Because these tend to be “medium-everything” soils (medium age, medium fertility, medium depth, medium pH, medium moisture, etc.) they do not actively dissuade human systems from occupying them with high population densities or intensity of management and production, especially as the global population increases. However these soils are often easily degraded, and so sustainable methods are especially important to guarantee future food production.

Finding out information on soils using the soil order suffix in the name of the soil according to the USDA soil taxonomy system.

Soil taxonomy is an enormous classification system that can initially be confusing. But knowing the first level of classification can be very useful, just like knowing whether an animal is a whale or a beetle is extremely helpful compared to not knowing anything. To classify soils broadly as to their limitations and productive potential, we can use the soil orders of the USDA system (see the order names in parentheses, in Fig. 3.8).

The key below will help you to use the last few letters of a USDA soil name, along with the ISRIC world soil mapping resource to query what types of soil are present around the world or specifically in your capstone regions. The categories are the same as what is presented in Figure 3.3, and you can use the query function in the ISRIC world soil mapper to find out what USDA soil names are present in each area, and draw conclusions about the potential fertility and properties of the soils at a broad level.

First, see the ISRIC resource is at SoilGrids [60]. This was also used in the formative assessment for Module 3.1.

In the ISRIC mapper you will need to click on layers icon in upper right and set the layer to “Soil Taxonomy: TAXOUSDA” and select the “All TAXOUSDA subclasses” -- when you query the map using a right click of the mouse, you’ll get a percent breakdown of the different soil orders at that location.

Key to USDA Soil Taxonomy System
Soil name ending Meanings Example
-Epts
-Ents
-Alfs
Entisols : soils of recent deposition, no soil development.

Inceptisols: the beginning of soil formation – medium to high fertility soils

Alfisols: broad class of medium age, medium to high fertility soils
Glossoboric hapludalfs

Orthents

-Ols Mollisols: prairie soils, high organic matter, generally neutral pH, fertile, deep Dystric haplustolls
-Ids Aridisols – dry region soils, generally high pH Argids
-Ods Spodosols – coniferous forest soils with acid needle litter leaching features Orthods
-Ults
-Oxes
Ultisols – warm region, old, leached soils

Oxisols – oldest tropical soils formed only of weathering remnants, metal oxides

Udults
-Ands Andisols- volcanic ash soils Vitrands
-Erts Vertisols – highly weathered limestone, with shrink-swell clays. Uderts

5 Nevertheless, you may peruse this impressive global resource and the soil horizon definitions at ISRIC [64].

Module 5.2: Soil Nitrogen and Phosphorus: Human Management of Key Nutrients

Introduction

Nutrient Cycling and Nutrient Management for Soils in Food Production

In module 5.2, we present a basic account of nutrient cycling and nutrient management in food production systems. When we talk about nutrients in this context, we are referring to the nutrients that are needed to grow crops which are taken up from soils by the roots of crop plants. These include the important nutrients nitrogen (N) and phosphorus (P) which will form the focus of this module. We refer to N and P as "important" nutrients because they are needed in large quantities, relative to the amounts that are readily available in many soils. In agricultural and ecological terms, we say that crops and food production are especially responsive to N and P abundance: shortage of N or P causes dramatic declines in production of food, while sufficiency and abundance will raise yields, so that N and P supply have been a focus of human management to maintain food production. We will begin by talking about the way that N and P move around in cycles in all ecosystems, including the agroecosystems that are managed by humans to produce food. Human management systems in agriculture thus play a major role in altering the cycles of these nutrients in order to maintain, and in some cases increase the production and supply of food from farmland (farmed soils). This management can also negatively impact water quality in watersheds, as you saw in module four. We will also understand the way that soil organic matter (SOM) relates to these two major nutrients and soil productivity, as well as the general concept of soil depletion and soil regeneration as these relate to strategies of soil management in food production.

What is Nutrient Cycling?

In module four, and in your education previous to this course, you've learned about the water cycle, in which water evaporates from bodies of water, condenses into clouds, and then is returned as rain to drain again into groundwater, lakes, and oceans. Each of the major crop nutrients, and most chemical elements on the earth's surface, has a similar cycle in which the nutrient is transported and transformed from one place to another, spending time in different 'pools', analogous to the division of water into lakes, rivers, clouds, rain, and the ocean. Just as rainwater and groundwater may be of more immediate use to crop plants than the ocean, different pools of the same nutrient differ in availability to plants. For example, most soils hold a tremendous amount of nitrogen in large organic molecules, but only the smaller soluble pool, and some smaller molecular forms of N, are directly available to plants. The way that soil nutrients move through the earth system, including within food production systems, is called nutrient cycling. The objective of this module is for you to understand the main features of nitrogen (N) and phosphorus (P) cycling in human-managed soils. Earth scientists sometimes use the term "biogeochemical cycling" to emphasize that each nutrient’s cycle represents the geological and atmospheric sources of the nutrients, the biology of organisms that often transform nutrients from one form to another, and the chemical nature and interactions of each element.

As an example of biogeochemical cycling, think of the important element carbon (C). Carbon has a chemical nature that allows it to be a fundamental molecular building block for all living things. In addition, there is an impressive atmospheric pool (a sort of geologic pool) of non-organic carbon dioxide. Interacting with this atmospheric pool, green plants and algae play a fundamental role in turning atmospheric CO2 into biological organic carbon in living things and the remains of living things, such as plants, that fall back into the soil. Scientists refer to this large set of interacting parts with geological, biological, and chemical attributes, earth's system that "processes" and recycles carbon in a certain sense, as the biogeochemical C cycle. Another example is phosphorus (P), which will be described in more detail on the following pages: The earth’s crust is the primary source of all P, which is then weathered by geological and biological processes and also in human fertilizer factories, held or retained strongly by soil clay minerals after application by farmers, and eventually occupies a key role in every living thing as one of the elements within the DNA molecules encoding our genes. It’s essential to realize that humanity and human systems are now major players within these nutrient cycles including C, P, and nitrogen. We can see this in activities such as mining (and eventually threatened depletion) of phosphorus sources for fertilizers, or fixing of large amounts of nitrogen for fertilizers with a massive expenditure of energy and emission of carbon dioxide through the use of oil and gas.

Soil Depletion and Regeneration: Human Management of Nutrients in Soils

The proper management of soil nutrients in soils for human food production boils down to a simple requirement: the need to replace nutrients that are "subtracted" from soil during production. These subtractions occur as nutrients are taken up by crops from the soil and then exported as food products in crops and livestock. Nutrients can also be lost to soil erosion and in dissolved forms, by drainage of water from the soil (called leaching). The goal of incorporating manure, plant material, and chemical fertilizers by farmers is to add back these subtracted nutrients. In the case of soil erosion, the idea is to avoid such losses completely by protecting soils. Human-managed fields and farms can be compared to nutrient bank accounts, where withdrawals must be balanced by deposits, and where it is better to have a substantial balance than a minuscule balance. Natural systems like forests or prairies lose some nutrients as does a farm field, but to a comparatively minor degree (fig 5.2.1 below). The need for humans to replenish nutrients is much greater in any managed system like a crop field or pasture than in unmanaged forests or grasslands. This is especially true in intensive production systems of crops or animal forages, for example, the corn, vegetable, and hay fields and pastured rangelands that are typical in agriculture of the United States and around the world. In systems where soils are tilled to grow annual crops on hillsides, the combined exported nutrients in food and those lost to erosion can quickly rob a soil of most of its nutrients. Protecting a soil from these losses, and regenerating the nutrients lost by adding crop residues (straw, cornstalks, other stems, and roots), manure, and fertilizer materials (ash, phosphate rock, bone, chemical fertilizers) are therefore important strategies used by food producers to sustain production. We’ll devote more focus to the important role of crop species, crop rotations, tillage, and soil erosion as part of agroecosystems in modules 6 and 7. For now, we want to understand the basics of these principles of soil regeneration.

Wild and crop production systems. See text above for more details.
Fig. 5.2.1: Nutrient cycling (biogeochemical cycling) in a closed natural ecosystem (left) and a crop production system (right) in which humans must replace exported nutrients from soils through the use of plant residues, manure, and other soil fertility inputs. The magnitude of soil nutrient losses in flows such as erosion, leaching, and nitrogen gas emissions from soils tends to also be greater in the cropped system than the losses from soils found in natural systems. The relationships of crop management to nutrient cycling will recur in greater detail in Module 7 with the concept of an agroecosystem.
Credit: Steven Vanek

Depletion and regeneration of soil organic matter

Soil Organic Matter as a Soil "Master Variable"

In addition to individual nutrients like N, P, potassium (K) and calcium, an overarching aspect of soil depletion and regeneration by human food producers is the important role played by soil organic matter (SOM) and the potential to either to deplete or sustain organic matter in soils (recall figure 5.1.1 and the fact that organic material is one of the key solid components of soil). In particular, concerns about soil organic matter (SOM) center on the large amounts of organic carbon in large molecules of SOM. This soil organic carbon (SOC) both feeds microbes in soil, allowing them to perform nutrient cycling functions and also contributes positively to soil properties. SOC is not a plant nutrient that comes from soil. In fact, it actually comes originally from the atmosphere in the form of plant remains that contain carbon fixed by plants (roots, leaves, manure, rotting wood etc.) and accompanies N, P, and other nutrients that were in the plants. SOC within soil organic matter plays so many important roles in soil function and soil fertility that it should be considered a “master variable” explaining soil productivity, along with soil pH, soil depth, and soil drainage. Among its other functions, SOM promotes soil storage of crop-available water, is a major food source for soil microbes that perform beneficial roles in soil, and fosters the availability of many nutrients by holding them in moderately available form or decomposing to release them in soils. In addition, by far the largest pool of nitrogen in soils is held in N atoms within many types and sizes of soil organic molecules, and also within the bodies of soil microbes.

In many food production systems where the soil is plowed (also called tilling or tillage), SOM is in fact depleted by oxidation (a “slow burn”, like iron rusting) when soils are broken apart by plows, hoes, and other implements. Therefore, an important part of soil regeneration by human food production systems is not just replacing nutrients in a pure chemical form like fertilizers, but also maintaining overall soil function with soil organic matter. Therefore, in most parts of the world farmers have developed ways of reincorporating the roots and stems of plants (crop residues) as well as manure made by animals from the forage crops fed to them. These sources of plant carbon sustain SOM over the long term and feed microbes. These ways of sustaining the nutrients and organic matter of soils are depicted with a coupled human natural systems diagram below (Fig 5.2.2) as a type of feedback loop in which human systems respond to soil degradation by incorporating organic matter like residues, compost, and manure.

The following brief reading assignment further illustrates the important functions of organic matter.

Reading Assignment

Building Soils for Better [56]Crops, [56] pages 9-17 in Chapter 2: Organic Matter: What It Is and Why It’s So Important. [56] (Free e-book). This chapter and book will be used in modules 7 and 9. Download at:

Activate Your Learning

The following exercise asks you to use graphical data based on real soils to make conclusions about the important role of SOM in water-holding capacity of soils. Along with the materials in module 4 on water and food production, and the systems approach to soil management in module 7, these concepts should help you to appreciate the role of SOM in fostering the environmentally sustainable production of food, as well as resilient systems (see module 10) that can deal with drought stress.

graph of soil organic matter and available water capacity
Fig. 5.2.2. Storage of crop-available water associated with the texture of soil (sandiness, clayiness) and its level of organic matter (SOM)
Credit: Steven Vanek, based on data from Hudson, B.D. 1994. "Soil organic matter and available water capacity. Journal of Soil and Water Conservation 49: 180-194.

Examine Fig. 5.2.2, which draws on about sixty soils analyzed in a publication that related the water-holding capacity of soils to their organic matter content. The graph summarizes that data as the height of three columns on a bar graph. The height represents the amount of water stored in each soil, imagined as a depth of water in mm covering the soil at its surface (this is also how irrigation managers imagine applying water to soils, as the mm of rainfall they have replaced with irrigation). Each column represents a type of soil, from a coarse-textured sand on the left to a "heavy" or clayey soil on the right. The stacked colors on the graph represent the way that organic matter is able to improve the water-holding capacity of soils. Answer the following questions (you can click on the box after answering to find out the answer).

In a dry climate, what type of soil would store the most water between rainfall events, regardless of soil organic matter level (a sand, a silt loam, or a clay)?


Click for answer.

ANSWER: the silt loam

How many mm of water storage approximately do you gain by increasing the soil organic matter level of a clayey soil from 1% to 3%?


Click for answer.

ANSWER: about 15mm

3. You talk to the manager of an experiment that is comparing a crop of maize grown in a field with a low-organic matter sand (1% SOM) and a high-organic matter silt loam (3% SOM), which miraculously are on either side of a farm lane. As you walk along next to the experiment, she tells you incredulously, "It's amazing, after this one-month drought, the silt loam maize is doing so well that it looks like it has had an extra imaginary rainstorm. It looks like about ______ inches of rain fell there that didn't fall on the maize in the sandy soil!"

If you had to fill in her blank you would guess: _______ inches (hint: remember to convert mm to inches, divide by about 25)


Click for answer.

ANSWER: two inches since the increase from 1% sand to 3% silt loam is about 50mm, and 1 inch = 25.4 mm

The Nitrogen Cycle and Human Management of Soils

Nitrogen (N) is one of the most important nutrients for plant growth and crop production, along with phosphorus (P) considered on the next page. Nitrogen is important because it is used by plants for to create proteins, which include the enzymes and building blocks of their photosynthetic "machinery". In fact, nitrogen in some ways underlies the green color of plants and vegetated areas on earth's surface, because of the green, N-containing chlorophyll proteins (enzymes) used in photosynthesis (see module 4), which along with the other photosynthetic enzymes is one of the major uses of nitrogen within plants. These plant proteins become animals protein when plants are fed to livestock, or when we eat plants. The ubiquitous nature of nitrogen for the protein needs of earth's biosphere explains why N is such an important nutrient for plant growth. Nitrogen is, therefore, a key element in the entire food system and interacts very strongly with human management. One indication of nitrogen's importance to the food system is that humans currently expend more energy on creating N fertilizers for food production by taking N2 out of the atmosphere in fertilizer factories (Fig. 5.2.3) than is spent on any other nutrient.

This module focuses on the subject of nutrient cycling, and below in figure 5.2.3, we present a basic diagram of the nitrogen cycle. Your initial impression of the diagram may be its relative complexity compared to the water cycle, for instance. This is true: the N cycle is complex, starting with the fact that it involves gas, solid, and liquid forms: gaseous N in the atmosphere, solid forms of N in soils and plants, and N dissolved in water in the soil and in earth's waterways (you may remember the problem of N pollution in waterways from module 4). To simplify this and take away the key concepts which should be your goal in this module (entire courses can be taught on the N cycle), we will present the basic pathway of N from the atmosphere into plants, soils, and water, which will complement the caption for the N cycling diagram below. Please refer to Figure 5.2.3 throughout this description. First, N exists in an enormous reserve as 78% of earth's atmosphere (top left of Fig. 5.2.3). Creating usable forms of nitrogen requires that this N2 gas is "fixed" in the same way that plants fix carbon into their carbonaceous stems and leaves. Legume plants like beans, peas, and alfalfa host bacteria in their roots in nodules that are able to fix N2 gas (more on legumes as an important crop family in module 6). Nitrogen then moves directly into legume plants' tissues as proteins. In parallel to this biological fixation of N, humans have designed industrial methods to fix N in factories, using energy from petroleum and natural gas, and creating soluble nitrogen chemicals that are applied to soil, where they dissolve in soil water to become part of the pool of soil soluble N that is available to plants. This pool of soluble N (light green oval within the soil N pool below) is also called inorganic N to contrast it from organic N in proteins, crop residues, and soil organic matter. Inorganic N taken up by plants, plus the N fixed by legumes, is then used to grow crops and eventually produce crop- and livestock- based food products. Meanwhile, organic "waste" products from growing crops like straw, cornstalks, and roots, plus animal manures which are undigested plants, are not "waste" at all but are a hugely important organic source of N and other nutrients that is recycled to soil (brown arrows in Fig. 5.2.3). These organic soil inputs applied by farmers help to maintain soil organic matter (SOM; see previous pages and the assigned reading on soil organic matter) including the largest pool of soil N within SOM and soil microbes. Soil organic matter can be decomposed by microbes, liberating additional amounts of N to the inorganic N pool. µbes also can take up soil inorganic N, reversing the effects of SOM decomposition.

Schematic showing main features of nitrogen (N) cycling in food production systems
Figure 5.2.3 Main features of nitrogen (N) cycling in food production systems. The diagram shows the multiple forms of N in soils and food production. Despite its complexity the diagram can be more simply considered in four parts: (1) a large atmospheric pool of N at upper left, which is used to make chemical fertilizer in factories and also by legumes to directly absorb N from the air for their own protein needs; (2) A soil pool which includes a predominant pool of organic N in large organic molecules (Soil Organic Matter or SOM) and microbes, as well as a fluctuating pool of inorganic N that is soluble in water (nitrate and ammonium ions); (3) The crops at the center of the diagram that are a main focus of human food production, and draw nitrogen from the pool of inorganic N in soil as well as from the atmosphere (in the case of legumes) and (4) Crop and livestock nitrogen exports from soil of nitrogen that then move through the food system to consumers. The very important return of crop residues and manure N to the soil N pool should also be noted (brown arrows). In addition, N can be lost from the soil in ways that are not productive (red arrows): as gases back to the atmosphere, including N2O, a potent greenhouse gas; as leaching losses of nitrate in rainfall into waterways, and as erosion of soil particles that include soil organic N that move out of agricultural fields, eventually becoming sediments in waterways and estuaries.
Credit: Steven Vanek

So far the N cycle may appear a relatively neat and ingenious system (albeit quite complex!). However, it is important to highlight the ways that it can become problematic under human management, indicated by the red "loss" arrows in Fig. 5.2.3. First, when the soluble N pool in soil is large, for example after fertilizer or manure is applied, and abundant water moves through the soil, like during a rain event, excessive soil N can move into waterways causing pollution and coastal dead zones (this is covered in some detail in module four, and again in this module's summative assessment). This process is called leaching of soil soluble N. Second, when erosion occurs, soils can also lose large amounts of their N "bank account" through erosion, because solid organic matter particles are rapidly eroded from soils in hilly areas when soil is not protected by plant cover or stabilized by plant roots. Lastly, soils can lose nitrogen back to the atmosphere through the processes of gaseous loss, where dissolved nitrogen becomes N-containing gases that diffuse back to the atmosphere. If you have ever caught a whiff of ammonia from a bottle of ammonia cleaning solution (dissolved ammonium that becomes ammonia gas) you know how N can move from a solution like that in a wet soil into the air. The most serious of these gas loss pathways is nitrous oxide (N2O) which is of concern because it is a potent greenhouse gas that contributes to global warming.

All of these loss pathways create the impetus for farmers and the food systems that support them, to manage nitrogen in an efficient and non-polluting way. The idea that highly productive farming systems with annual crops, manures, and fertilizers can completely eliminate N losses is actually quite challenging. This is because the N cycle has so many participants (humans, plants, microbes, livestock) interacting in complex ways (note: a complex system!), and because nitrogen is inherently "flighty" and "leaky", never staying put and always in transformation, with some forms so easily lost from soils to rivers, lakes, and the atmosphere. Nevertheless, there is much room for improvement that can also serve to save money and energy for food producers, and avoid the pollution costs to downstream ecosystems and food producers (for example, fishing communities affected by dead zones, see module 4). Two of these are (1) increasing the efficiency of timing and amounts of N fertilizer and manures to better match only what is needed by crops and (2) including crops and other plant components on farms that help to recycle soluble N from deeper in the soil and in downslope areas before it reaches waterways. Both of these strategies are addressed in the following modules on crops and systems approaches to soil management (modules 6 and 7). In addition, if N is not replenished in soils after it is exported as food products or suffers these losses, crops can face N insufficiency, which is a major issue for poorer farmers around the world. The summative assessment for this module focuses on these twin issues of nutrient deficiency and excess.

The Phosphorus Cycle and Human Management of Soils

Basics of The Phosphorus Cycle in Food Production

In an analogous way to the nitrogen (N) cycle on the previous page, we will present the basics of the phosphorus (P) cycle related to food production (refer to figure 5.2.4 below in this section) You'll note that the P cycle is a good deal simpler than the N cycle. For example, there is no gaseous form of P as there is for N, so the atmosphere does not participate in the P cycle. Also, leaching of soluble P is not a major issue as it is for soluble soil N. To begin the description of the P cycle, the large reserve of "primary" P that is accessed by plants and fertilizer production for agriculture is not the atmosphere (as it is for N), but rather so-called phosphate rocks (or rock phosphate) in the crust of the earth, which are mined like other minerals. These rocks are ground up and treated in fertilizer factories to make the phosphate (PO4-) in them water-soluble so that phosphate can be directly taken up by plants from the small pool of soluble phosphorus in soils. In addition to this industrial process that supplies P to plant roots, there are small amounts of soluble P that are continually released by weathering (see Module 5.1) of grains of rock phosphate that form a small part of most soils. These plant-available forms of P from fertilizers and weathering are taken up by plants and pass into the food system when crops are harvested for food products or are fed to livestock. Just as for N (figure 5.2.3), crop residues and manures with organic P are recycled to the soil and are an essential way of replenishing soil organic P supplies. Also, decomposition of soil organic P that liberates soluble P, and uptake of P into the bodies of microbes, link the organic P pool in soil organic matter (SOM) with the small amount of soluble P in soils.

Fig. 5.2.4. Diagram of phosphorus (P) cycling relevant to food systems, drawn in an analogous way to the N cycle in figure 5.2.3. This diagram can be thought of as four main areas: (1) Phosphate rock deposits that are used to produce phosphorus fertilizers (along with "native" grains of rock phosphate in soils, see below); (2) Soil P, which is divided into a larger organic P pool and a relatively minuscule pool of soluble P, and also contains relatively unavailable forms called "retained P" held on soil minerals like clays; (3) Phosphorus in plants, which includes all tissues and the energetic machinery of plants; (4) Crop and livestock P exports to the food system. Note that there are multiple flows into and out of soils of P, and also fractions and internal cycling within soils from one form of P to another. In addition to the rock phosphates that are mined and turned into fertilizer, many soils also have their own mineral P supplies in the form of small rock grains that contain rock phosphate. These amounts are small but can be sufficient in wild ecosystems (Fig. 5.2.1) under normal weathering processes that release nutrients in most soils (see module 5.1 for a description of weathering). The different fractions of soil P represented by the shapes in the soil box are not necessarily in proportion to their relative size; also the relative amounts in different pools varies considerably among different soils.
Credit: Steven Vanek

P retention in soils and management responses: "clingy P" versus "flighty N"

One difference between the cycling of P vs. N in soils is the fact that most soils have ways of chemically capturing and holding soluble P in forms that can become very unavailable to plants. The clay mineral fraction of soils is especially active in retaining P, especially so for the clays that occur in tropical soils (you may be familiar with rusty or yellow-colored clays, made from iron oxides, in warmer areas of the United States and the world). This is called soil retention or fixation of P. In a soil that retains P strongly, less than five percent of the P in applied fertilizer, which enters in a soluble form very suited for plant uptake, is ever available for crops. The rest is quickly locked away by reactions with soil clay minerals. Soil scientists call this process P fixation or P retention, and a global map of estimated P retention has been made (Figure 5.2.5) that summarizes how phosphorus can become limiting to food production, which is a serious problem in many tropical soils. One comparison that may be helpful in remembering the way that soil locks away phosphorus is to contrast it to the behavior of soil N. While soil N is "flighty" or "leaky" with multiple forms and loss pathways, soil P tends to be the "clingy" opposite of soil N -- the issue is not that it is held too loosely in soils but rather that it is held too tightly.

To address the challenge of retained P, farmers may resort to continually supplying fertilizers and manures to crops, often in quantities that greatly exceed crop demand. Nevertheless, additions of organic matter also tend to make retained or fixed P more available, combined with the use of crop species that can better take up fixed forms of P, so that P is moved from the retained, unavailable fraction of P to organic forms in crop and microbial biomass that are eventually recycled into available soluble forms. Certain plant-symbiotic soil microbes, especially mycorrhizal fungi, are particularly efficient at helping plants to access these less soluble forms of soil P. In addition to these soil management measures, first farmers, and now formal plant breeders have developed crop varieties that are more efficient in taking up some of retained P that is locked away in soil.

Fig. 5.2.5. Global map of soil P retention potential. Old and very old soils in warm-climate areas, as well as leached soils under cold-climate conifer vegetation, tend to exhibit the highest rates of soil P retention.  These soils can make applied P fertilizers very unavailable to crops. In response, farmers can apply lime to raise the pH of these often acid soils, or continually re-supply P via fertilizers (which can be very inefficient if most of this fertilizer is quickly made unavailable); or add P in organic forms that become solubilized by decomposition and can directly feed plant roots before P is fixed. In practice, all of these approaches are used.
Image Credit: United States Department of Agriculture, Natural Resource Conservation Service

Soil Erosion and P

As can be seen in Fig. 5.2.4, erosion of particles of soil that contain organic and retained P is the major pathway of phosphorus loss from soils (red arrow in Fig. 5.2.4), in contrast to P export for useful purposes in crop- and livestock-based foods. Along with maintaining the availability of soil P with regard to P retention, protecting soils against erosion is an excellent way to protect the ability of soils to supply P for food production. This main message will be taken up in the summative assessment for this module, and again in Module 7.

Activate Your Learning: phosphorus nutrients required for different foods: a per-acre versus per-person approach

One of the important factors in deciding how much P must be added to soils to replenish them is the amount of P that is exported by typical crops and food products. This exercise will guide you in calculating the amounts of P that leave farm fields on a per area basis, and also at the level of a "phosphorus use footprint" for typical products, analogous to a water footprint in module 4. Consider the table below which reports the use of P to produce unit quantities of a few representative foods. The first column (A) reports very approximately how much a single hectare of soil (100 by 100 m area, about 2.5 acres) will support. The second column (B) is the content or concentration of phosphorus in the food, which means that multiplying A x B gives the kg P exported from the soil by the crop or animal product, which is shown in C. Columns D and E take a slightly different approach: in D the amount of the product eaten by an average U.S. person is reported. In column E, that per-person amount is turned into a per-person consumption of phosphorus in grams (per year)

Phosphorus used to Produce Unit Quantities of Some Representative Foods
Food crops

(A) kg of fresh product or animal weight sustained per hectare (100 m x 100 m)

(B) Phosphorus content of the fresh food (g P/ kg fresh wt.) (C) kg P exported from soil, per Ha (100 m x 100 m) (D) Per person consumption of product in the U.S. (kg per person per year) (E) Per capita consumption of soil P resources (g P per person per year)
carrots 10000 0.35 3.5 3.2 1.1
wheat 3500 7.6 27 61 464
beef 250a 7 1.8 50 350
milk 10000b 3.6 36 20 72

Table assembled by the author from publically available data on typical yields and nutrient content of agricultural products. For example for yield data see National Agricultural Statistics Service (NASS) of the USDA [65] for crop nutrient content see National Resource Conservation Service's Crop Nutrient Database [66]. For nutrient values of foods such as beef and milk see the USDA food composition database [67].

aAbout the equivalent weight of half a beef cow/steer
bVery roughly a single production cycle (about 12 months) in liters for a single, lactating cow of a high-production variety

Questions:

Of the food crops carrot or wheat, which crop removes more phosphorus from the soil on a per-Ha or per-acre basis?

Click for answer.

ANSWER: wheat with 26.6 kg P/ hectare (additional note: crops that export grain, seeds, or flowers (e.g. wheat, beans, broccoli) from soils usually export more P than those that export just stems or leaves (e.g. kale) or roots (e.g. carrots).

Of the animal products, which product removes more phosphorus from the soil on an average annual basis per person?

Click for answer.

ANSWER: Beef with 350g P per person per year. (of course for people who do not eat beef the number is zero, but this figure is an average across beef eaters and non-beef eaters)

As is sometimes the case for complex systems, this simple table hides some complexity and "hidden flows" for phosphorus associated with meat and milk production. The numbers used here reflect the milk or beef that leaves an average farm and goes to consumers, called a farm-gate flow because it measures what passes out of a farm to consumers. Unlike the crops, where the only phosphorus used comes from the soil (which can include fertilizers applied to the soil), animal products reflect both the P in the beef or milk that left the farm but also additional P that may have come from other farms in the form of feed crops. This additional P was fed to the animals but not used in the growth of the animal or milk production. Where does that P end up? What is done with that P? (hint: think smelly!)

Click for answer.

ANSWER: mostly in manure, which is spread back onto fields, and can, therefore, build up in agricultural soils.

Soil nutrients: Human Systems Aspects

Soil Nutrients: The Sustainability Issues of Shortage and Surplus

Both N and P are distinctive in possessing extremes of surplus and shortages across the variety of food production systems around the globe. For poorer small-scale farmers, who number more than two billion globally, the means to effectively replenish the nutrients exported by crops, or detain the nutrients removed by erosion on sloping land can be beyond the reach of their financial means or labor power, or simply not sufficiently part of their knowledge systems. Deficits of nitrogen and phosphorus in soils ensue, complicated by soils that may have a high degree of P retention, and low organic matter levels that decrease the overall soil quality by retaining less water and crusting easily, aspects that will be emphasized in the following modules. Applying the "bank account" analogy of soil nutrients introduced at the beginning of this module, after constant withdrawals the "soil bank account" begins to run such a low balance that overall functioning of soil productivity, and with it the livelihood of a smallholder household, are impaired. This can lead to a downward spiral of soil productivity (see the assigned reading for this module) that links issues of environmental, social, and economic sustainability.

Fig. 5.2.5. The concept of the downward or vicious cycle of soil nutrients and organic matter as a driver of soil productivity (brown spiral), and a "virtuous cycle" alternative (green spiral) where the maintenance of soil nutrients and soil quality broadly with organic matter, allows for better livelihoods and reinvestment in soils with nutrients. We will revisit this diagram in module 10 when talking about sustainable food systems.
Credit: Steven Vanek

Another feature of human-natural interactions for soil nutrients is the aspect of surplus exhibited by a "leaky" or "flighty" nutrient like nitrogen. This has been compounded by the development of large-scale human capacity to add surplus nutrients to farm fields for food production. It's important to realize that prior to N fertilizers, bacterial nodules on the roots of legume crops (see Fig. 5.2.3 and the coverage of legumes in module 6) were the major way that N entered soils from the atmosphere, including the soils used for food production. Farmers before about 1900 relied exclusively on legume crops, as well as animal (and human!) manures derived from legumes and other crops as the principal way of regenerating the nitrogen in soil organic matter. These materials incorporated to soils decompose and release N that was used by crops. Since 1913, when N fertilizer production from the atmosphere was developed as a factory process, humanity has deployed greater and greater amounts of fossil fuel energy to fix greater and greater amounts of atmospheric N2 into soluble forms to feed crops. A startling fact is that humans now fix more atmospheric nitrogen than do legumes. This has buoyed the overall productivity of human food systems beyond what might have occurred without such fertilizers and is credited by many with avoiding widespread hunger (or dramatically expanding the population carrying capacity of earth’s human-natural systems, depending slightly on the perspective taken).

As has been noted in module 4, there have been unforeseen consequences of this trend towards greater fertilizer use that have become more evident in recent years. First, the share of CO2 greenhouse gas emissions from fertilizer production has become a primary contributor of the overall impact of agriculture on global warming. Another is that fertilizers, in combination with a profit-minded vision of soil fertility that did not incorporate a view of the whole human-environment system, bred a highly “chemical” vision of soils that neglected the important role of soil organic matter and the physical and biological qualities of soil. This resulted in unforeseen negative impacts as farmers over-applied nutrients at a local scale to guarantee the highest yields possible, thereby polluting watersheds, and allowing farmers to lose sight of the important role of soil organic matter outlined in this module. In a more subtle way, there has been an increasing focus in plant breeding and globalized seed systems on varieties that respond well to soluble fertilizers, which many argue have favored the expansion of more industrial modes of food production to the financial detriment of smaller and more sustainable food producers. If you recall the narration of agricultural history in module 2, you will recognize that this is an example of niche construction, in which a modern, chemical-intensive niche has been created for specially bred modern varieties along with fertilizers and other chemical inputs. Nevertheless, many of these problems associated with an exclusive reliance on nitrogen fertilizers and chemical fertilizers are now recognized by researchers and policymakers. Current approaches to soil the world over have placed renewed emphasis on the importance of organic matter and a more economical use of nitrogen fertilizers.

In the summative evaluation for this module, you will explore these “surplus and shortage” issues of sustainability for Nitrogen and Phosphorus, which are emblematic of present-day and future sustainability challenges in the area of nutrients cycling.

Summative Assessment

N and P Balances

Introduction

The last page of module 5.2 mentions the twin issues of deficit and surplus that are principal challenges in the management of soil nutrients. The exercise in this summative assessment requires you to use real data on nutrient inputs and outputs from two systems to create nutrient balances, and then analyze the situation of nutrient balance or surplus. These systems are the Ohio River sub-basin of the Mississippi River basin and measurements of nutrient flow from hillside farming in the Bolivian Andes. You should do this activity with a partner or small group in class, and prepare to discuss your results with the class. You will use data from a table to answer questions on the assessment worksheet (download below).

In analyzing the twin issues of nutrient surplus and nutrient shortage in soils and food production systems, you'll be practicing a geoscience "habit of mind" of systems thinking. In other words, to examine the wider impacts of nutrient management or the causes of soil infertility, we need to expand our focus from a single field to a landscape or river basin and think about a web of linkages between farmers, nutrient supplies, economic factors, and watersheds, among other system components. This allows us to contemplate these challenges in the proper frame and over the right timescale.

Download the worksheet [68] to complete and turn in your assessment. The worksheet contains information in a table that you will need to complete the assignment.

Submitting your Assessment

For on-line course: After completing the worksheet, please take the assessment quiz in Canvas. For hybrid course: please submit your assignment in Canvas.

Grading Information and Rubric (not applicable for online course; grading rubric for in-class assessment activity).

Your assignment will be evaluated based on the following rubric. The maximum grade for the assignment is 38 points.

Rubric
Item Possible Points
Completeness of filling in balance terms and response, correct use of data and calculation of balance 5 per balance
Short answer section, questions 2, 4, 6 through 9 12
Question 10, 2 points per answer a-e: evidence of applying concepts from this module and previous modules to the two nutrient management cases 10
Overall grammatical correctness, spelling, complete sentences 6

Summary and Final Tasks

Summary

In this module, we have introduced the basics of soil properties and the nature of soil as a key resource for food production, which following modules will build upon to show how soils can be managed sustainably. We hope that you have understood the fundamental composition of soil as minerals, organic matter, water, and air as an essential part of earth's natural systems.We also have tried to illustrate the way in which key properties of soil, like its pH, nutrient content, and retention of water, affect how plants grow and produce food. On the human system side, we also presented the way in which human efforts have managed soil for sustained production of food, including the addition of nitrogen and phosphorus to replenish soil stores that are removed by crop harvests, and the protection of soils from erosion losses. However, a surplus of soil nutrients generated by over-applying N and P is also a problem, as illustrated in the nutrient balances in this module's summative assessment. We will continue to deepen your knowledge of sustainable soil management, as it supports sustainable food systems, during the next modules.

Reminder - Complete all of the Module 3 tasks!

You have reached the end of Module 5! Double-check the to-do list on the Module 5 Roadmap [69] to make sure you have completed all of the activities listed there before moving on to Module 6!

Further Reading

  1. Brady, N.C. and Weil. R.R. 2016. The Nature and Properties of Soils. Columbus: Pearson. Very readable and visual textbook that gives an extremely comprehensive treatment of soil science.
  2. Dybas, C.L.,2005 "Dead Zones Spreading in World Oceans [70]" Bioscience 55(7): 552-557 - freely available article in Bioscience journal.
  3. Scoones, I. (2010). Dynamics and Diversity: soil fertility and farming livelihoods in Africa, case studies from Ethiopia, Mali, and Zimbabwe. Earthscan.

Module 6: Crops

Overview

Agroecosystems are coupled Human-Nature Systems that are shaped by ecological and human socioeconomic factors.

Agricultural practices that humans use are determined by multiple agroecological factors including climate, soil, native organisms, and human socioeconomic factors. Usually, climate and soil resources are the most significant natural factors that determine the crops and livestock that humans produce. Although in some cases, to overcome climate and soil limitations, humans alter the environment with technology (ex. irrigation or greenhouses) to expand the range of food and fiber crops that they can produce. In this module, we will explore how climate and soil influence crop plant selection; crop plant characteristics and classifications; and some socioeconomic factors that influence the crops that humans chose to grow.

Goals and Learning Objectives

Goals

  • Describe key features of categories of crop plants and how they are adapted to environmental and ecological factors.
  • Explain how soil and climatic features determine what crops can be produced in a location, and how humans may alter an environment for crop production.
  • Classify environments as high or low resource environments and interpret how both environmental and socio-economic factors contribute to crop plant selection (coupled human-nature systems); and the pros and cons of the cultivation of various crop types.

Learning Objectives

After completing this module, students will be able to:

  • Define annual and perennial crops and list some examples of annual and perennial crops.
  • Distinguish and explain why annual or perennial crops are cultivated in high resource or resource-limited environments.
  • Explain some ways that farmers alter the environment to produce annual or perennial crops.
  • Name some major crop plant families with some example crops.
  • Explain the nutrient significance of legumes.
  • Describe key plant physiological processes and how climate change may influence crop plant growth and yield.
  • Classify major crop plants into types including plant families, temperature adaptation, and photosynthetic pathways.
  • Formulate an explanation of the advantages and disadvantages of producing annual and perennial crops.
  • Interpret what environmental, ecological, and socioeconomic factors influence what crops farmers produce.
  • Distinguish some environmental, ecological and socioeconomic advantages and disadvantages of producing types of crops.

Assignments

Module 6 Roadmap

Detailed instructions for completing the Summative Assessment will be provided in each module.

Module 6 Roadmap
Assignment Location
To Read
  1. Materials on the course website
  2. Virginia Cooperative Extension: The Organic Way - Plant Families
  3. Penn State Extension: Seasonal Classification of Vegetables
  4. Plant & Soil Sciences eLibrary: Transpiration: Water Movement through Plants
  5. National Climate Assessment Report: Introduction and Section 1 Increasing Impacts on Agriculture
  6. USDA: Background: Corn
  1. You are on the course website now.
  2. Online: The Organic Way - Plant Families [71]
  3. Online: Seasonal Classification of Vegetables [72]
  4. Online: Transpiration: Water Movement through Plants [73]
  5. Online: Increasing Impacts on Agriculture [74]
  6. Online: Background: Corn [75]
To Do

  1. Formative Assessment: NASS Geospatial Map Crop Scape Annual and Perennial Crop Analysis and Interpretation of Advantages and Disadvantages
  2. Summative Assessment: Top 15 World Food Commodities
  3. Take Module Quiz
  4. Turn in Capstone Project Stage 2 Assignment
  1. In course content: Formative Assessment [76]; then submit in Canvas (as a discussion post)
  2. In course content: Summative Assessment [77]; then take quiz in Canvas
  3. In Canvas
  4. In Canvas

Questions?

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Module 6.1: Crop Life Cycles and Environments

Climate, soil resources, and the organisms in the environment influence which food and fiber crop plants humans can produce. To overcome environmental resources limitations, humans also alter the environment to produce food and fiber crops.

Plant life histories

Plants need light, water, nutrients, an optimal temperature range, and carbon dioxide for growth. In a natural environment, the availability of plant resources is determined by the:

  • soil fertility, soil depth, and soil drainage
  • climate: the seasonal temperature and precipitation distribution
  • competition with other plants, herbivory by other organisms, and pathogens
  • the frequency of environmental disturbances (for example from fire, floods, and herbivory).

In some environments, nutrients, light, and water, are readily available and temperatures and the length of the growing season are sufficient for most annual crops to complete their lifecycle; we will refer to these as high resource environments for crop production. High resource environments tend to have soils that are fertile, well-drained, deep, and generally level, as well as growing seasons with temperatures and precipitation that are optimal for most plant growth. In general, in environments where competition for resources among plants is low, annual plants with more rapid growth rates tend to dominate (Lambers et al, 1998). Consequently, humans tend to cultivate annual plants with high growth rates in high resource environments.

By contrast, in low-resource environments plant growth may be limited due to soil features and/or climatic conditions. Soils may be sloped, with limited fertility, depth, and drainage; and/or the growing season may be short due to extended dry seasons and/or long winters (with temperatures at or below freezing). In natural ecosystems, resources can be limited due to competition among plants, such as in a forest or grassland where established plants limit the light, water, and nutrients for new seedlings. And in these environments where resources are limited, plants with slower growth rates and perennial life cycles tend to succeed (Lambers et al, 1998), and perennials are often the primary crops that humans cultivate in resource-limited environments. 

Annuals

Annual plants grow, produce seeds, and die within one year. In general, annual plants evolved in environments where light, water, and nutrients were available, and they could consistently reproduce in one year or less. Where resource availability is high, plants that can germinate and grow rapidly have a competitive advantage capturing light, nutrients, and water over slower growing plants and are more likely to reproduce. To ensure the survival of their offspring, annuals allocate the majority of their growth to seeds (often contained in fruit); and they tend to produce many seeds.

Human selection of annual crop plants typically further selected for large seeds and/or fruit. Some examples of annual crop plants are corn, wheat, oats, peppers, and beans (see photos). What are some other examples of annual crop plants?

Cornfield
Figure 6.1.1: Cornfield
Credit: Heather Karsten
Wheat field
Figure 6.1.2: Wheatfield
Credit: Heather Karsten
Corn
Figure 6.1.3: Corn Grain
Credit: Heather Karsten
Tomatoes and peppers
Figure 6.1.4: Peppers
Credit: pixabay [78]
Beans
Figure 6.1.5: Beans
Credit: pixabay [79]

Annual crop plants are generally categorized into one of the three seasons that falls in the middle of their plant growth life cycle: spring, summer, or winter. For instance, summer annuals are generally planted in late spring, grow and develop through summer, and complete their lifecycle by late summer or autumn. Winter annuals are generally planted in early autumn and germinate and grow in autumn. Depending on how cold the winter is where they are cultivated, winter annuals may grow slowly in winter or become dormant until spring. In spring, they grow, flower, and produce seed by early to mid-summer (See Figure 6.1, Annual Crop Types). After an annual crop is harvested, in some regions farmers may be able to plant another crop, such as a winter annual crop after a spring annual crop, this is referred to as double-cropping (cultivating two crops in one year). If only one crop is cultivated in a season, the soil may be left exposed until the next growing season. Leaving crop residue on the soil can reduce erosion, but planting another crop with live plant roots and aboveground vegetation provides better soil protection against water and wind erosion. Alternatively, a cover crop may be planted after the harvested crop to protect the soil from erosion and provide other benefits until the next crop is planted. Cover crops are typically annual crops that can establish quickly; you will learn more about cover crops in Module 7.

Seasons annual crop types with their approximate growing seasons in Northeastern US
Figure 6.1.6: Annual seasonal crop types with their approximate growing seasons in Northeastern US
Credit: Heather Karsten

Perennials

Biennials are plants that live and reproduce in two years, and at the other end of the life cycle spectrum are perennial plants that live for 3 or more years. Perennials evolved in environments where resources were limited often due to competition with other plants and their growth rates tend to be slower than annual plants (Lambers et al, 1998). In these resource-limited environments, often plants cannot germinate from seed and reproduce by seed within one year. Therefore, to increase their opportunities for successful reproduction, perennials evolved ways to grow and survive for multiple years to successfully produce offspring. Perennial crops are typically cultivated in environments that may also have a climatic limitation such as a short growing season or dry climate, or where plant ability access to resources may be limited due to frequent disturbance such as grazing.

To survive for multiple years, perennials allocate a high proportion of their growth to vegetative plant parts that enable them to access limited resources and live longer. For instance, they often invest in extensive and deep root systems to access water and nutrients, or in tall and wide-reaching aboveground stems and shoots to compete for light, such as bush and tree trunks and branches. Perennials also store reserves to regrow after growth limiting conditions such as drought, freezing winters, or disturbance such as grazing. Carbohydrates, fat, and protein are stored in stems and roots, or modified stems such as tubers, bulbs, rhizomes, and stolons. In many plant species, these storage organs can produce root and shoot buds that can grow into independent offspring or clonal plants; this is called vegetative reproduction. Although most perennials reproduce both through seed and vegetative reproduction, in resource-limited environments where plant competition is high, the large storage organs and their reserves offer vegetative offspring plants a competitive advantage over starting from seed.

Perennial Crops

Humans have cultivated and selected perennial crop plants for their vegetative plant parts, storage organs, fruit, and seeds. For instance, the leaves and stems are the primary plant parts harvested from perennial forage crops (crops in which most of the aboveground plant material is grazed or fed to animals). Horticultural perennial crops that are harvested for stems and leaves include asparagus, rhubarb, and herbs. And in some cases, a perennial crop's storage organs are harvested each year, limiting the plant's ability to complete its perennial lifecycle and effectively reducing its cultivated lifecycle to an annual. Examples of such crops perennial crops that are cultivated as annuals include potato, sweet potato, and taro, Tree, shrub, and vine food crops managed as perennial crops are typically cultivated for their fruit and seeds, such as apples, stone fruit (ex. peach, plum), plantains, nuts, berries, and grapes (see photos below).

Perennial alfalfa
Figure 6.1.7. Perennial alfalfa field in Pennsylvania.
Credit: Heather Karsten
Two men in an Apple orchard
Figure 6.1.8. Apple orchard in Washington. Entomologist Brad Higbee (left) with Jerry Wattman, manager of this apple orchard near West Parker Heights, Washington.
Credit: Scott Bauer of USDA
potatoes
Figure 6.1.9. Potatoes in a supermarket in Lima, Peru.
Credit: Heather Karsten
Fruit on a table at open air market
Figure 6.1.10. Perennial tree crops at an open-air market in Peru.
Credit: Heather Karsten
cows grazing mountains in background
Figure 6.1.11. Foreground: Perennial grassland grazed by Angus cattle in western Montana, where precipitation and high daytime temperatures in summer limit plant growth. In the background, perennial grasses and broadleaf plants including shrubs and trees on the foothills of the mountain range have deep roots to access soil moisture that also help reduce soil erosion.
Credit: Heather Karsten
Rangeland in Utah
Figure. 6.1.12. Rangeland in Southwest Utah where precipitation and high temperatures limit plant growth
Credit: Heather Karsten

Check Your Understanding

What are some other perennial crop plants?


Click for answer.

ANSWER: Perennial crop plant photos (alfalfa, perennial grasses, alfalfa roots, potatoes, raspberries, bananas, oranges)
Alfalfa roots
Figure 6.1.13. Alfalfa roots
Credit: Kulbhushan Grover

Crop Life Cycles and Environments

Annual plants are typically cultivated in high-resource environments and regions with:

  • climates that have sufficient precipitation and temperatures for plants to complete their life cycle each year
  • soils that soils tend to be relatively flat and well drained, and are not prone to erosion when they are tilled or planted to an annual crop each year
  • high fertility soil

Annual crops produce grain and fruit crops within one growing season. Grain crops are typically a concentrated source of carbohydrates, protein and sometimes fat, that can be cost-effectively stored and transported long distances, enhancing their market options and utility. Grain and oilseed annual crops are often processed for multiple uses and markets. For instance, oil is extracted from soybean for industrial and human uses, and the remaining meal is high in protein that is used for both human food products and livestock feed.

If conditions are not ideal for annual crops, farmers sometimes use management practices or technologies to improve conditions for crop growth such as irrigation to compensate for the lack of precipitation or black plastic to warm the soil in environments where temperatures may limit plant growth.

Check Your Understanding

What are some other examples of practices or technologies that farmers might use to increase annual crop production?


Click for answer.

Examples include: soil amendments such as fertilizer, lime, organic matter amendments (compost, manure, cover crops); season extension solar-heated hoop houses, or greenhouses; shorter season crop varieties, greenhouses, tile drains to improve soil drainage

Regions, where perennial crops dominate the landscape, tend to have soil or climatic limitations such as steep or hilly slopes that are prone to erosion, shallow or poorly drained soils, soil nutrient limitations; limited precipitation and soil moisture availability, short growing seasons, or temperatures outside of optimal plant growth temperatures. In these environments, farmers may produce annual crops that are adapted to the environment, such as spring or winter wheat that grow during the cooler season or drought-tolerant annuals such as sorghum and pearl millet. Or farmers may use technologies and management practices, particularly for high-value crops, to improve conditions for crop growth such as tile drains, irrigation or season extension technologies.

See illustration and comparison of plant life cycles, the time and forms of reproduction. Can you name a specific crop plant example for each type of plant life cycle?

Weed life cycles
Figure 6.1.14. Illustration and comparison of plant lifecycles.
Credit: Bellinder, R. R.; R. A. Kline, D. T. Warholic. 1963. Weed control for the Home Garden [80]. Cornell Cooperative Extension Bulletin 216. Figure 1, Pg. 6.

Perennials and Soil Conservation

Because perennials allocate a high proportion of their growth to vegetative structures and regrow for many years, they can: i. protect soil from erosion; ii. return organic matter (carbon-based materials that originated from living organisms) to the soil, providing multiple soil health benefits; and iii. remove carbon dioxide from the atmosphere, potentially sequestering (storing) carbon in the soil or aboveground plant biomass. Forests, for example, sequester carbon above-ground in trees and in below-ground root systems.

Perennial grasses, in particular, have dense, fibrous roots that protect soil from erosion well and are valuable plants for soil conservation. In addition, over the years, some perennial roots and aboveground plant tissues die when environmental conditions limit growth (ex. drought, winter, grazing), and accumulate organic matter and nutrients in the soil. The majority of the most fertile and deep agricultural soils of the world were formed under natural perennial grasslands, whose deep root systems accumulated organic matter in the soil which contributed many beneficial soil properties, as well as carbon sequestration. Some annual crops can also contribute to conserving soil and add organic matter to the soil if a large portion of the crop residue is left on the soil surface, such corn stalks left on a field after the grain is harvested.

Cows grazing
Figure 6.1.15. Perennial grass roots, belowground rhizomes, and aboveground plant tissues provide year-round protection from soil erosion on a sloping field in Pennsylvania, while also providing forage for ruminant dairy cows during the spring, summer and autumn months.
Credit: Heather Karsten
Perennial Grasses
Figure 6.1.16. Some cool-season perennial grasses with rhizomes indicated by the red arrows.
Credit: Maria Carlassare
perennial grass
Figure 6.1.17. Perennial grass mowed and drying for hay harvest on a steeply sloped field in Pennsylvania.
Credit: Heather Karsten
Sheep in New Zealand
Figure 6.1.18. Sheep moving to high mountain pastures in the New Zealand South Island of where perennial grasses and legumes protect the soil from erosion on steep slopes and the perennial life cycle is adapted to the short growing season.
Credit: Heather Karsten

Check Your Understanding

Short Answer

Can you name some well-known, high-value perennial crops that are produced in the mountainous regions on the steep slopes of the following countries: Switzerland, Costa Rica, Columbia, Peru, Italy? Type them in the space below.


Click for answer.

ANSWER:
Answers could include forage crops for animals that produce milk and meat, coffee, chocolate, cashews, avocados, bananas, plantains, oranges, potatoes, olives, grapes, etc.

Formative Assessment

NASS Geospatial Map Crop Scape Annual and Perennial Crop Analysis and Interpretation of Advantages and Disadvantages

Instructions

Use the US Department of Agriculture (USDA) National Agricultural Statistics Survey geospatial map CropScape [81] to view the distribution of agricultural crops in the US. On the left side of the online map, click on the Cropland Data Layers tab and select the most recent year; then click on the Legend tab at top of the left side of the map to see what crop species each color represents. You can scroll down the long legend list to find the many crop species shown on the map.

Next, use the many tools at the top of the map to select more detailed information. When you hover over the symbols in the top menu, the text explains what each symbol does will appear. You can zoom in or select a state of interest using the USA flag-colored map symbol and then view the summary statistics for the selected region or state by clicking on the bar graph symbol.

Using the US flag-colored map symbol, select one state from the following two sets (A and B): A. Illinois or Indiana and B. Vermont or Wyoming. Look at the map and then click on the bar graph symbol and review the list of crops or crop types and the estimated acres of production. Be sure to scroll down the list of crop types to review them all, as they aren't all visible in the list window. Note that some crops are summarized into a category of similar crop types.

  1. What are the two crops or crop types that are produced on the most acres in each of the two states that you selected? Are the crops annuals or perennials?
  2. Distinguish and discuss how the soil characteristics of each state classify as a high-resource or resource-limited environment. Use the SoilGrids maps and soil class descriptions that you viewed earlier in Soils and Nutrients Module to describe the primary US soil order (of the 12 USDA soil orders) in the states you selected. See SoilGrids [82]. See also Plant & Soil Sciences eLibrary [83], for a general description of the USDA soil order that dominates the state you selected. In addition, view the topographical map of the US, found at ArcGIS [84], to describe the topography and degree of slope of most agricultural regions in each of the two states. See: 
 [84]Does the topography explain the dominant crop types in each state?
  3. How does the climate differ between the two states you selected? View the following websites to view the average temperature and precipitation in the US and the two states that you selected. Select average normals [85] and Western Regional Climate Center [85]. In addition, compare the plant growth hardiness zones for the two states you selected. See USDA Agricultural Research Services [86]. How does the climate explain which crops dominate in each of the two states you selected? Is it likely that farmers are using technologies or management practices to alter the environment to create high-resource conditions for the dominant crops produced in your states, such as irrigation, greenhouses, etc.?
  4. Based on what you have learned about annuals and perennials, differentiate two potential advantages and disadvantages for a farm growing annuals and perennials. Write a paragraph that analyzes and interprets the pros and cons of both annual and perennial crops. Consider how they are adapted to different environments, how each crop type impacts the soil and potential pests dynamics, as well s the socioeconomic benefits and disadvantages of annual and perennial crop cultivation.

Answer the above four questions in about 1500 words.

Download the worksheet [87] to complete the activity

Submitting Your Assignment

Please submit your assignment through Canvas

Grading Information and Rubric (not applicable for online course; grading rubric for in-class assessment activity)

The maximum point for this assignment is 38. Use the Grading Rubric below to better understand what you will be graded on.

Grading Rubric
Criteria Possible Points
1. In two selected states, the two dominant crops are identified and their life cycle is correctly identified. 6 points
2. The answer offers a logical interpretation of how the soil types and topography of each state characterize as a high or low-resource environment and explain what crops are dominant. Answers should link to the concepts discussed in the module and the Soils and Nutrients Models (3 points for each state). Incomplete or incorrect answers will be scored lower. 6 points
3. The answer summarizes the average temperature and precipitation for each state, and the crop hardiness zone rating and the climate contribute to classifying as a high or low resource environment. (4 points for each state) Answers should link to the concepts discussed in the module and water module. 8 points
4a. Analysis and interpretation of 2 advantages of growing annuals and 2 advantages of growing perennials for a farm and farmer are accurately and clearly described.(2 points for each advantage). Incorrect or incomplete answers will be reduced by 1 or 2 points. 8 points
4b. Analysis and differentiation of 2 disadvantages of growing annuals and 2 disadvantages of growing perennials for a farmer are accurately and clearly described (2 points for each disadvantage). Incorrect or incomplete answers will be reduced by 1 or 2 points. 8 points
5.Writing is grammatically correct, clear and well organized. 2 points

Module 6.2: Crop Plant Characteristic Classification and Climatic Adaptations

In addition to their lifecycles, crop plants are characterized and classified in multiple ways that are relevant for crop production and management. Common plant features include similar morphology, growth and reproduction; and environmental and climatic adaptions. This module will help you understand more about how crops are adapted to different environments and diversified to interrupt pest lifecycles.

Plant Families

Plants that have similar flowers, reproductive structures, other characteristics, and are evolutionarily related, are grouped into plant families (See Figure 2). Species in the same plant family tend to have similar growth characteristics, nutrient needs, and often the same pests (pathogens, herbivores). Planting crops from different plant families on a farm and the landscape; and rotating crops of different plant families over time can interrupt the crop pest life cycles, particularly insect pests, and pathogens, and reduce yield losses due to pests. Increasing plant family diversity can also provide other agrobiodiversity benefits including, diverse seasonal growth and adaptation to weather stresses such as frosts, and drought; different soil nutrient needs, as well as producing diverse foods that provide for human nutritional needs.

Enter image and alt text here. No sizes!
Figure 6.2.1: The Plant Family Tree
Credit: The U. S. Botanic Garden and the National Museum of Natural History, Department of Botany, Smithsonian Institution [88].

Read this summary of the major world food crop plant families and the value of knowing what family plants are in, The Organic Way - Plant Families [71], then consider these questions.

  1. What plants are your five favorite foods produced from?
  2. What plant families are they in?
  3. Are they annuals or perennials?

The Fabaceae/Leguminosae, commonly called the Legume plant family, is important for soil nitrogen management in agriculture and for soil, human and animal nutrition. Legume plants can form a mutualistic, symbiotic association with Rhizobium bacteria which inhabit legume roots in small growths or nodules in the roots (seed images in the video listed below). The rhizobia bacteria have enzymes that can take up nitrogen from the atmosphere and they share the “fixed nitrogen” with their legume host plant. Nitrogen is an important nutrient for the plants and animals, it is a critical element in amino acids and proteins, genetic material and many other important plant and animal compounds. Legume grains crops, also called pulses are high in protein, such as many species of beans, lentils, peas, and peanuts. Most of their plant nitrogen is harvested in grain, although there is some in crop residues that can increase soil nitrogen content. Perennial legume crops are typically grown as forage crops for their high protein for animals. Because they allocate a large portion of their growth to vegetative plant parts and storage organs, perennial legumes also return a significant quantity of nitrogen to the soil, enhancing soil fertility for non-legumes crops grown in association or in rotation with legumes.

Watch the following NRCS video about legumes and legume research.

Video: The Science of Soil Health: Understanding the Value of Legumes and Nitrogen-Fixing Microbes (2:30)

Click for video transcript.
Legumes and cash and cover crops use natural symbiotic relationships with soil microbes to get nitrogen into the soil. NC State University's Dr. Julie Grossman is working to provide farmers with new insights on how to harness this resource. My work really involves looking at legumes to try to figure out how we can really make them make them the most efficient nitrogen source we possibly can, by looking at the microbial component of the legume-rhizobia symbiosis. And we work a lot with organic farmers simply because right now, that's those are the farmers who are really interested in using legumes for nitrogen supply. As nitrogen prices go up we're gonna need to turn to some of these alternative processes such as nitrogen fixation. And when that happens, we need to be able to hit the ground running. We can't say, “okay now we're gonna start doing the research.” We really want to get to know how, when you take a bacteria, a strain of bacteria, and you look at its DNA, how does it differ from other strains of bacteria. Because you can have some that are very high performers and they fix a lot of nitrogen and you can have others that don't really do a heck of a lot for the plant. In my mind, what would really help the farmers is trying to understand the tools they can use as farmers to help increase nutrient supply to their crop plants. So try to figure out how much nitrogen is supplied when they put a legume in the soil and let it decompose, how that is released when it's released, how we can get more nitrogen into the legume by enhancing the fixation ability of the microbes. So all these little pieces will help us be able to help farmers develop their own research, their own experimentation, so they don't need to rely on the recipes. They can say, “Oh, I know that if I can calculate a square meter of legume biomass and I can calculate how much I have and how much nitrogen is in that square, I can then figure out on my whole field, how much nitrogen is being added through this legume to my soil.” And so those are the kinds of things I really want to give to farmers, in terms of having them understand how they can control their own biological process, in their fields on their own, and not have to rely on recipes.

Activate Your Learning

What are some of the benefits of including legumes in a crop rotation?


Click for answer.

ANSWER: Legumes return some nitrogen to the soil, particularly perennial legumes that allocate a proportion of their resources to vegetative growth (ex. roots and shoots) and storage organs. Legumes also produce a crop that is high in protein, whether it is a high protein annual grain legume (also called a pulse) or perennial crop.

Plant Classification Systems and Physiological processes

In addition to characterizing plants by their taxonomic plant family, crop plants are also classified as either cool season or warm season, referring to the range of temperatures that are optimum for their growth. Examples of cool-season agronomic crops include wheat, oats, barley, rye, canola, and many forage grasses are called cool-season grasses, such as perennial ryegrass, timothy, orchardgrass, tall fescue, smooth bromegrass, and the bluegrasses. Warm season agronomic crops include corn or maize, sorghum, sugarcane, millet, peanut, cotton, soybeans, and switchgrass.

Learn more about the differences in cool and warm season plants and the types of vegetable crops in these categories by reading Season Classification of Vegetables [89].

In addition, plants are classified by the type of photosynthetic pathway that they have.

Plant Photosynthesis, Transpiration, and Response to Changing Climatic Conditions

Plants require light, water, and carbon dioxide (CO2) in their chloroplasts, where they create sugars for energy through photosynthesis. The chemical equation for photosynthesis is:

6 CO2+ 6 H2O → C6H12O6+ 6 O2

Carbon dioxide (CO2) enters plants through stomata, which are openings on the surface of the leaf that are controlled by two guard cells. The guard cells open in response to environmental cues, such as light and the presence of water in the plant.

Enter image and alt text here. No sizes!
Figure 6.2.2: Stomate on a Tomato leaf
Credit: Wikimedia Commons, Tomato leaf stomate [90], Public Domain

For a brief and helpful review of photosynthesis and plant anatomy such as the plant leaf structures, see Plant Physiology - Internal Functions and Growth [91].

Water (H2O) enters the plant from the soil through the roots bringing with it important plant nutrients in solution.

Transpiration or the evaporation of water from plant contributes to a “negative water potential.” The negative water potential creates a driving force that moves water against the force of gravity, from the roots, through plant tissues in xylem cells to leaves, where it exits through the leaf stomata. Since the concentration of water is typically higher inside the plant than outside the plant, water moves along a diffusion gradient out through the stomata. Transpiration is also an important process for cooling the plant. When water evaporates or liquid water molecules are converted to a gas, energy is required to break the strong hydrogen bonds between water molecules, this absorption of energy cools the plant. This is similar to when your body perspires, the liquid water molecules absorb energy and evaporate, leaving your skin cooler.

Diagram of water molecule leaving stomata - side view
Figure 6.2.3: Picture of water molecules leaving stomata - side view
Credit: USDA National Institute of Food and Agriculture [92], found at Plant & Soil Sciences eLibrary [93]

Carbon dioxide (CO2) also diffuses into the plant through the stomata, because the concentration of carbon dioxide is higher outside of the plant than inside the plant, where carbon dioxide concentration is lower due to plant photosynthesis fixing the carbon dioxide into sugars. To conduct photosynthesis, plants must open their leaf stomata to allow carbon dioxide to enter, which also creates the openings for water to exit the plant. If water becomes limited such as in drought conditions, plants generally reduce the degree of stomatal opening (also called “stomatal conductance”) or close their stomata completely; limiting carbon dioxide availability in the plant.

Schematic of gas exchange across plat stomata
Figure 6.2.4: Schematic of gas exchange across plant stomata
Credit: ASU School of Life Sciences, Snacking on Sunlight [94]

Read more about how water moves through the plant and factors that contribute to water moving into the roots and out of the plant, as well as carbon dioxide movement in Transpiration - Water Movement through Plants [73].

Check Your Understanding

After completing the above reading about transpiration and factors that contribute to water entering and leaving the plant, answer the following questions:

How does transpiration influence the temperature of the plant?


Click for answer.

Answer: Transpiration or water evaporation from the plant stomata cools the plant, protecting important plant enzymes and plant processes, such as photosynthesis and pollen formation.

In many regions, climate change projections are for warmer air temperatures which will likely increase evapotranspiration (the loss of water as a gas from soil and plants). This will likely contribute to soil drying. If soil water is limited, plants tend to reduce their stomatal opening or close stomates to conserve water. How will reduced transpiration impact the plant's physiological ability to cool or avoid overheating?


Click for answer.

Answer: If transpiration stomatal opening is reduced evaporative cooling will be reduced and a plant temperature will increase.

C3 and C4 photosynthesis

The majority of plants and crop plants are C3 plants, referring to the fact that the first carbon compound produced during photosynthesis contains three carbon atoms. Under high temperature and light, however, oxygen has a high affinity for the photosynthetic enzyme Rubisco. Oxygen can bind to Rubisco instead of carbon dioxide, and through a process called photorespiration, oxygen reduces C3 plant photosynthetic efficiency and water use efficiency. In environments with high temperature and light, that tend to have soil moisture limitations, some plants evolved C4 photosynthesis. A unique leaf anatomy and biochemistry enables C4 plants to bind carbon dioxide when it enters the leaf and produces a 4-carbon compound that transfers and concentrates carbon dioxide in specific cells around the Rubisco enzyme, significantly improving the plant’s photosynthetic and water use efficiency. As a result in high light and temperature environments, C4 plants tend to be more productive than C3 plants. Examples of C4 plants include corn, sorghum, sugarcane, millet, and switchgrass. However, the C4 anatomical and biochemical adaptations require additional plant energy and resources than C3 photosynthesis, and so in cooler environments, C3 plants are typically more photosynthetically efficient and productive.

Since carbon dioxide is the gas that plants need for photosynthesis, researchers have studied how the elevated CO2 concentrations impact C4 and C3 plant growth and crop yields. Although C3 plants are not as adapted to warm temperatures as C4 plants, photosynthesis of C3 plants is limited by carbon dioxide; and as one would expect research has shown that C3 plants have benefitted from increased carbon dioxide concentrations with increased growth and yields (Taub, 2010). By contrast, with their adaptations, C4 plants are not as limited by carbon dioxide, and under elevated carbon dioxide levels, the growth of C4 plants did not increase as much as C3 plants. In field studies with elevated carbon dioxide levels, yields of C4 plants were also not higher (Taub, 2010). In addition, if soil nitrogen was limited, C3 plant response to elevated CO2 concentration was reduced or crop plant nitrogen or protein content was reduced compared to plants grown in high soil N conditions (Taub, 2010). These results suggest that crops will likely require higher soil nutrient availability to benefit from elevated atmospheric carbon dioxide concentrations. For more optional reading information about C3 and C4 plant response to elevated carbon dioxide concentrations, see the following summary of research that is also listed in the additional reading list, Effects of Rising Atmospheric Concentrations of Carbon Dioxide on Plants [95].

Other Drought Tolerant Crop Plant Traits

Some additional plant traits that help plants tolerate drought and heat stress include deep root systems (typical of perennials) and/or thick leaves with waxes that reduce water loss and the rate of transpiration. In addition, some plants roll their leaves to reduce the surface area for solar radiation reception and heating, and some reduce their stomatal conductance more (water loss) more than others.

Temperature

Elevated temperatures projected with climate change can have multiple impacts on plant growing conditions. Climate change may lengthen growing seasons in some regions, although day lengths will not change. As planting dates are altered with longer growing seasons, crops may also be exposed to high temperature, moisture stress, and risk of frost. Elevated temperatures may also increase evaporation of water from the soil, reducing soil water availability. Higher temperatures are not necessarily ideal for yield, even if the temperatures are below a plants’ optimal temperature. At elevated temperatures, plants grow faster which tends to, one, reduce the amount of the time for photosynthesis and growth, resulting in smaller plants, and two, reduce the time for grain fill, reducing yield, particularly if nighttime temperatures are high (Hattfield et al., 2009). High temperatures can also reduce pollen viability, be lethal to pollen. The multiple effects of high temperatures on plant physiological process and soil moisture likely explain why research has found that grain development and yield are often reduced when temperatures are elevated (Hattfield et al., 2009).

Many factors that are projected to change with climate change could influence plant growth. These include carbon dioxide concentration, temperature, precipitation and soil moisture, and ozone concentrations in the lower atmosphere.

Read the Introduction and Key Message 1 (Increasing Impacts on Agriculture) of the National Climate Assessment [96].

Check Your Understanding

How will multiple climate change factors that are projected to change together (such as temperature, carbon dioxide concentration, and soil moisture availability) likely to differ influence crop plant growth and yields?


Click for answer.

Answer: Although an increase in carbon dioxide has the potential to increase plant productivity in some plants, such as C3 plants, in many cases the combination of elevated temperature and ozone, and reduced soil moisture availability are likely to outweigh the increased availability of C02 and result in reduced crop yields.

Agricultural Crops Case Studies

Socio-economic Factors

In addition to the climate and soil resources for crop production, many socio-economic factors influence which crops farmers chose to cultivate, including production costs, domestic and international market demand; and government policies that subsidize agricultural producers, and reduce trade barriers or export costs. As discussed in Module 3, the protein, energy, fat, vitamins and micro-nutrients of crops for human nutrition are one predictor of the market value of a crop. However some food crops are highly-valued and cultivated for their cultural and culinary qualities, such as flavor (ex. chilies, vanilla, coffee, wine grapes); and their high economic value often reflects high production and processing costs, as well as market demand for their unique culinary and cultural properties.

Some crops are cultivated for non-human food uses such as livestock feed, biofuel, fiber, industrial oil and starch, and medicinal uses. Crop processing often creates by-products that can be used for other purposes, adding market value. For example, when oil is extracted from oilseeds such as soybean, the soybean meal by-product is high in protein and sold for livestock feed or added to human food products. And for crops that are cultivated on many acres often with support from government policies, the consistent, abundant supply of these commodity crops has contributed to the development of multiple processing technologies, uses, and markets. To better understand factors that contribute to the production of commodity crops, we will now examine two case studies of corn and sugarcane.

Understanding Agricultural Commodities: Two Agricultural Crops Case Studies 

In the following two agricultural crop case studies, you will have the opportunity to apply your understanding of crop plant life cycles, classification systems, and crop adaption to climatic conditions to understand how plant ecological features and human socioeconomic factors influence which crops are some of the major crops produced in the world.

Corn (Maize) Case Study

Enter image and alt text here. No sizes!
Figure 6.2.5: Cornfield in Pennsylvania
Credit: Heather Karsten

Corn or maize is a summer annual C4 crop in the Poaceae, or grass family that has high nutrient demands. Unless soil conservation practices are used, corn fields do not have live roots protecting the soil from erosion and providing other soil quality benefits after harvest in the fall, winter and spring. The US is the largest corn producer in the world. Soils and climate, particularly in the Midwest, permit high corn yields; and significant investment in agricultural research has produced high-yielding corn hybrids and production technologies, such as fertilizers, pest control practices, farming equipment, and irrigation. Research has also developed diverse uses for the large quantities of corn produced in the US, and the US is also a major exporter of corn.

Activate Your Learning

Read this overview of US corn production and uses from the US Department of Agriculture, Economic Research Service, Corn and Other Feed Grains [97]. Then answer the following questions:

1. What are two top uses of corn in the US?


Click for answer.

ANSWER: livestock feed and for ethanol production for energy

2. Discuss at least three specific factors that explain why the amount of corn produced in the US has increased over the past 30 years?


Click for answer.

ANSWER: Demand for ethanol has increased the price of corn. Yields have increased with improved production technologies (seeds, pest management, tillage practices etc.). The export market continues to grow. Growing US population from Latin America has increased demand for corn for food.

Sugarcane Case Study

The US consumes the most sweeteners of any country in the world. In the US, high-fructose syrup is made from corn, which has displaced some sugarcane production for sugar for US market. Sugarcane production, however, has continued to increase in Brazil, the biggest sugarcane producer in the world. Sugarcane is a C4 perennial crop in the grass family and its not  grown for just for sugar as a food sweetener.

Activate Your Learning

Watch this United Nations video below, about the factors contributing to increased sugarcane production and some of the consequences. Then answer the questions below.

Video: Brazil: The ethanol revolution (4:55)

Click for video transcript.
49 year-old Severino Ramos de Enraja works for Moema mill, a large agribusiness company in Sao Paulo state in southeastern Brazil. From sugarcane the company makes sugar and ethanol alcohol, which partially substitutes for gasoline in Brazil. Less gasoline means reducing the harmful pollution which is changing the world's climate. But despite his work, Severino and hundreds of thousands of others may end up losing their jobs, ironically due to the success of their industry. I'm getting old and I don't have an alternative. I hope to be able to find work elsewhere. Tadeo Endraj is a director at the country's leading scientific research and development center. No other country has so much technology related to sugarcane. From producing plant varieties, growing, cutting, transporting, and other industrial processes related to sugar and alcohol production. During the 1970s, Brazil's economy was severely affected by an oil embargo and rising prices. The country's military government launched a national program to reduce its dependency on foreign oil. It encouraged the construction of ethanol plants, offering low interest loans to sugar companies and subsidies to keep the price of fuel low. The automobile industry adapted quickly. The widespread use of ethanol has made the country a global leader in cutting emissions and oil imports at the same time. Increases in world prices of oil, international tensions, and an urgent need to address environmental concerns, are fueling the rapid expansion of the international market for Brazilian biofuels. During the first six months of 2007, the country's ethanol exports shot up by 70%. This is the industry's future. Here at Moema Mills, 50% of the sugar cane harvest is already mechanized. The three workers that operate each of these machines can replace sixty cane cutters. The mechanization process is here, it has arrived. It's whirring for us. But can cutters themselves are your machines. We are the beginning of the entire process. Mechanized cutting is also seen as better for the environment. Traditionally, manual harvesting sugarcane is aided by burning, which clears the plants serrated leaves and tops. The burning is carefully controlled, but this wasn't always the case. Fires themselves create pollution and uncontrolled blazes have led to the destruction of forests and wildlife. State legislators have set a deadline for stopping this practice. By the year 2014, burning will no longer be permitted and almost all of San Paulo sugar plantations will shift from manual to mechanized harvesting. This means, cane cutters will no longer be needed. There are no guarantees that jobs will be found for each cutter, but there is awareness that mass unemployment could lead to social chaos. Ricardo Brito Pereira is Moema Mills’ director. They need social stability and we need to create employment. The cutters will be absorbed in our future expansion. This is our responsibility. It's not only up to the government, the unions, we have to be involved. Brazil aims to double its current production of ethanol in 10 years. This might mean more need for farm machinery. Many believe that the conversion of ethanol into a tradable commodity worldwide, as oil is, is crucial for lifting the developing world out of poverty. To balance environmental concerns, technological developments and the redeployment of hundreds of thousands of cane cutters, will be a major challenge for Brazilian society. This report was prepared by Heine Teskey for the United Nations.

If the video does not play, please see Brazil: The ethanol revolution (United Nations) [98].

Explain two factors that explain why sugarcane production has increased in Brazil.


Click for answer.

ANSWER: The government has encouraged the development of an ethanol for fuel to reduce dependency on foreign oil. They have encouraged the sugarcane ethanol industry through low-interest loans to sugar companies, government subsidies, and funding research. A foreign export market developed for ethanol due to oil prices, political interests in alternatives to oil and renewable energy. It also provides an economic incentive for sugarcane ethanol.

Summative Assessment

Top 15 World Agricultural Commodities

Instructions

For this summative assessment, you need to have completed the corn and sugarcane agricultural crops case studies in this module. If you have not, go back and read the linked ERS USDA website and watch the FAO video. Then go to FAO (Food and Agriculture Organization) FAOSTAT [99] and look up the top 15 agricultural commodities in the world in 2013, or the most recent year that data is available, as well as the top agricultural commodities in 2000. Download the ranking and total production of the top 50 commodities for 2000 and the most recent year. In a spreadsheet calculate the percentage of change in production of the most recent year's top 15 commodities then answer the below questions. Your instructor may provide you with this downloaded data and the change in production calculation. Analysis and critical thinking about the data are encouraged.

Answer the following questions:

  1. Describe the crops that are used to produce the top 15 agricultural commodities with the classification systems you have learned in Module 6.
    1. In what plant families are they?
    2. Are the top agricultural commodities produced from annual or perennial plants or both?
    3. Are they cool season, warm season, C3 or C4 plants?
  2. Which four commodities have increased in production the most in comparison to the other top 11 commodities, which had the greatest percentage of increase in production? By what percentage has the production (in weight, not dollars) of the top four agricultural commodities in the most recent year for which data is available changed since 2000?
  3. Why has corn production in the US and sugarcane production in Brazil increased recently? What markets, agroecological and socioeconomic factors do the case study readings and FAO video explain have contributed to the increased production of corn in the US and sugarcane in Brazil?
  4. What might socioeconomic, agricultural, and environmental factors explain the significant increase of the four commodities that increased most since 2000 on a global scale?
  5. Consider how the increased production of these four commodities likely impacts the soil, nutrient cycling, pest populations, and ecology of an agroecosystem? What are the potential pros and cons of these crops on soil, nutrient cycling, greenhouse gases, other ecological impacts; What are the socio-economic impacts? Distinguish the most significant impacts, and discuss why there are significant advantages or disadvantages of the expansion of these top 4 commodities. The pros and cons may be socioeconomic and/or environmental.

Files to Download

FAO Top 20 Commodity Changes Key spreadsheet [100]

Download the worksheet [101] to submit your assessment

Submitting Your Paper

Submit your paper online in the folder provided in your course management system.

Grading and Rubric

This assignment is worth a total of 55 points. Please read over the rubric to understand how the assignment will be graded.

Rubric
Criteria Possible Points
List the top 15 agricultural commodities from the FAO website. (each commodity worth 1/5 point) 5 points

Accurate description of the crop types used to produce the commodities, plant family, the crop life cycle, temperature, C3/C4 classifications (1/4 point for accurate identification of the four crop characteristics for each crop commodity)

30 points
Answer accurately and quantitatively describes if and how the production of the top commodities has changed since 2000 and which 4 commodities have increased the most and quantitatively how much production has changed. Up to 4 points may be deducted for incomplete or inaccurate responses. 4 points

Answer accurately explains what factors explain the increased production of corn in the US and sugarcane in Brazil. The explanation links to the Module concepts and earlier modules. Analysis and critical thinking are accurate. Up to 4 points may be deducted for incomplete answers that are not detailed or not grounded in Module and course concepts,

4 points
Answer offers a rationale and clear explanation for commodity production change of the four crops with the most significant increase in production that links to the Module concepts and earlier modules. Analysis and critical thinking are accurate. Up to 4 points may be deducted for incomplete answers that are not detailed or not grounded in Module and course concepts, 4 points
Analysis and selection of the potential advantages and disadvantages of changes in production of the 4 crops are accurately and clearly described. Full credit answers should provide a rationale for which pros and cons are most significant, and integrate information from this Module, and other course modules (1-2 points for each pro and con and the rationale for why they are important). Inaccurate or incomplete pros and cons will be reduced by 3, 2 or 1 point. 8 points
Answers are grammatically correct, well organized and clear. Up to 5 points may be deducted for poor writing.

Summary and Final Tasks

Summary

After completing Module 6, you should now be able to:

  • Describe key features of categories of crop plants and how they are adapted to environmental and ecological factors
  • Explain how soil and climatic features determine what crops can be produced in a location, and how humans may alter an environment for crop production.
  • Describe some plant physiological traits and differences that could influence how plants adapt to climate change.
  • Explain how both environmental and socio-economic factors contribute to crop plant selection (coupled human-nature systems).

Reminder - Complete all of the Module 6 tasks!

You have reached the end of Module 6! Double-check the to-do list on the Module 6 Roadmap [102] to make sure you have completed all of the activities listed there before moving on to Module 7.1!

References and Further Reading

Sterling, T. M. Transpiration in the Plant and Soil Sciences ELibrary: https://passel.unl.edu/pages/informationmodule.php?idinformationmodule=1... [73]

Taub, D. 2010. Effects of Rising Atmospheric Concentrations of Carbon Dioxide on Plants. Nature Education Knowledge 3(10):21

Lambers, H. S.Chapin and T. Pons. 1998, Plant Physiological Ecology. 2nd edition. Springer-Verlag New York. pg. 340 and 344.

Capstone Project Stage 2 Assignment

Water, soils, and crops

(Modules 4-6)

The diagram below summarizes the topics you will explore in Stage 2 for your assigned region. In Stage 2 of the capstone, you will engage in spatial thinking and geographic facility to interpret spatial data (for example annual precipitation, evapotranspiration and soils data) and interpret how multiple regional factors contribute to determining which crops are produced in your region.

Capstone Stage 2 Diagram
Capstone Stage 2

What to do for Stage 2?

  • Download and complete the Capstone Project State 2 Worksheet [103] that contains a table summarizing the data you’ll need to collect to complete this stage. Remember, you need to think deeply about each response and write responses that reflect the depth of your thought as informed by your research.
  • Add relevant data to your PowerPoint file.

  • Add questions and continue to research the questions in your worksheets.

  • Continue building a CHNS diagram to illustrate the connections between the natural system and the human food systems of the region. You may decide that you need multiple diagrams.

Capstone Project Overview: Where do you stand?

At this stage, you should have started to investigate your assigned region and have added information, maps and data to your worksheets and PowerPoint file for Stages 1 and 2.

Upon completion of stage 2, you should have at this point:

  • Continued research and data compilation in the Stages 1 and 2 tables in the associated Stages 1 and 2 worksheets.
    • Stage 1: Regional food setting, history of regional food systems, diet/nutrition
    • Stage 2: Water resources, soils, and crops
  • Added to your powerpoint file containing the data that you are collecting about the food system of your assigned region.
    • The information you may have:
      • Labeled map of your region
      • Soil map of your region
      • Precipitation and temperature map of your region
      • Major crops and crop families grown in your region
  • Continued to record citations for all references and resources you are using in your research. This is a critical step. Every figure, map, piece of data, and bit of information you collect from the web, a book, a person, a journal or any other source must be attributed to the source.
  • Added to your list of questions you have about your region related to key course topics and initiated significant efforts to answer.
  • Revised your CHNS diagram and/or create a new one incorporating topics from Modules 4, 5 and 6.

Source URL: https://www.e-education.psu.edu/geog3/node/820

Links
[1] https://www.epa.gov/sites/production/files/2015-09/documents/ag_runoff_fact_sheet.pdf
[2] https://www.e-education.psu.edu/geog3/node/1099
[3] https://www.e-education.psu.edu/geog3/node/1191
[4] https://www.youtube.com/watch?v=U26Na9Xa5Ug&amp;feature=youtu.be
[5] https://commons.wikimedia.org/wiki/File:Photosynthesis.gif
[6] https://creativecommons.org/licenses/by-sa/3.0/deed.en
[7] http://pacinst.org/publication/california-agricultural-water-use-key-background-information/
[8] http://nationalmap.gov
[9] http://12.000.scripts.mit.edu/mission2017/solutions/engineering-solutions/rainwater-harvesting-techniques/
[10] http://www.ers.usda.gov/topics/farm-practices-management/irrigation-water-use/background.aspx
[11] https://pubs.usgs.gov/circ/1405/
[12] http://waterfootprint.org/en/water-footprint/
[13] http://waterfootprint.org/media/downloads/Hoekstra-Mekonnen-2012-WaterFootprint-of-Humanity.pdf
[14] http://www.angelamorelli.com/water/
[15] https://www.e-education.psu.edu/geog3/sites/www.e-education.psu.edu.geog3/files/Mod4/Geog3_Module4_Formative2.docx
[16] http://waterfootprint.org/en/resources/interactive-tools/personal-water-footprint-calculator/
[17] http://www.unesco.org/new/fileadmin/MULTIMEDIA/FIELD/Venice/pdf/special_events/bozza_scheda_DOW04_1.0.pdf
[18] http://water.usgs.gov/edu/wuir.html
[19] http://www.fao.org/fileadmin/templates/solaw/images_maps/map_3.pdf
[20] https://water.usgs.gov/edu/wugw.html
[21] http://water.usgs.gov/edu/wugw.html
[22] https://water.usgs.gov/edu/gwdepletion.html
[23] http://bioscience.oxfordjournals.org/content/50/9/746.full.pdf
[24] https://www.e-education.psu.edu/geog3/sites/www.e-education.psu.edu.geog3/files/Mod4/Ag_Runoff_Fact_Sheet.pdf
[25] https://www.youngfarmers.org/
[26] http://water.epa.gov/type/watersheds/named/msbasin/images/eutro_big.jpg
[27] https://oceantoday.noaa.gov/happnowdeadzone/
[28] https://oceanservice.noaa.gov/hazards/hypoxia/
[29] https://pubs.usgs.gov/fs/2010/3078/
[30] http://ks.water.usgs.gov/pubs/fact-sheets/fs.135-00.html
[31] https://water.usgs.gov/nawqa/headlines/nut_pest/USGS-Trends-Sprague.pdf
[32] http://www.gulfhypoxia.net/
[33] https://coastalscience.noaa.gov/
[34] http://pubs.usgs.gov/fs/2003/fs-105-03/
[35] https://www.epa.gov/nutrientpollution/sources-and-solutions-agriculture
[36] http://www2.epa.gov/nutrientpollution/sources-and-solutions-agriculture
[37] https://coastalscience.noaa.gov/research/stressor-impacts-mitigation/habhrca/ngomex/
[38] https://www.epa.gov/sites/production/files/2015-03/documents/hypoxia_reassessment_508.pdf
[39] https://www.e-education.psu.edu/geog3/sites/www.e-education.psu.edu.geog3/files/Mod4/Module4_Summative_Revised092518.docx
[40] https://www.e-education.psu.edu/geog3/sites/www.e-education.psu.edu.geog3/files/Mod4/Module4_Summative_Revised092518PDF%202.pdf
[41] https://www.e-education.psu.edu/geog3/sites/www.e-education.psu.edu.geog3/files/Mod4/Module4_SummativeResults.xlsx
[42] https://www.e-education.psu.edu/geog3/606
[43] http://pubs.usgs.gov/fs/2010/3078/
[44] http://www2.epa.gov/sites/production/files/2015-03/documents/hypoxia_reassessment_508.pdf
[45] http://www.usbr.gov/lc/region/programs/crbstudy/finalreport/index.html
[46] http://www.arcgis.com/home/item.html?id=0698165058384852b23f31b26ae7cace
[47] https://agwaterconservation.colostate.edu/ag-water-faqs/crop-water-use/
[48] http://waterfootprint.org/en/resources/interactive-tools/product-gallery/
[49] https://youtu.be/h23IHDOKhZc
[50] https://youtu.be/j9JywZGtXA4
[51] https://youtu.be/U26Na9Xa5Ug
[52] https://youtu.be/64W2yh4RsgI
[53] http://www.fao.org/nr/water
[54] https://www.sare.org/Learning-Center/Books/Building-Soils-for-Better-Crops-3rd-Edition
[55] http://www.sare.org/content/download/841/6675/Building_Soils_For_Better_Crops.pdf?inlinedownload=1;
[56] http://www.sare.org/content/download/841/6675/Building_Soils_For_Better_Crops.pdf?inlinedownload=1
[57] https://www.e-education.psu.edu/geog3/node/863
[58] https://www.e-education.psu.edu/geog3/node/875
[59] https://www.flickr.com/photos/soilscience/
[60] http://soilgrids.org
[61] https://nelson.wisc.edu/sage/data-and-models/atlas/maps/anntotprecip/atl_anntotprecip.jpg
[62] https://www.e-education.psu.edu/geog3/sites/www.e-education.psu.edu.geog3/files/Mod3/Formative5.1_soilTransects_Amended_May2017.docx
[63] https://www.nrcs.usda.gov/wps/portal/nrcs/site/soils/home/
[64] http://www.isric.org/
[65] https://www.nass.usda.gov/
[66] https://plants.usda.gov/npk/main
[67] https://ndb.nal.usda.gov/ndb/
[68] https://www.e-education.psu.edu/geog3/sites/www.e-education.psu.edu.geog3/files/Mod5/Summative5.2_NandP_Editing_Jan2017.docx
[69] https://www.e-education.psu.edu/geog3/node/583
[70] http://bioscience.oxfordjournals.org/content/55/7/552.full.pdf+html
[71] https://www.e-education.psu.edu/geog3/sites/www.e-education.psu.edu.geog3/files/Mod6/OrganicWay.pdf
[72] http://extension.psu.edu/plants/gardening/fact-sheets/vegetable-gardening/seasonal-classification-of-vegetables
[73] https://passel.unl.edu/pages/informationmodule.php?idinformationmodule=1092853841&amp;topicorder=1&amp;maxto=8&amp;minto=1
[74] http://nca2014.globalchange.gov/report/sectors/agriculture
[75] https://www.ers.usda.gov/topics/crops/corn-and-other-feedgrains/background/
[76] https://www.e-education.psu.edu/geog3/node/830
[77] https://www.e-education.psu.edu/geog3/node/997
[78] https://pixabay.com/en/vegetables-pepper-tomato-chayote-607305/
[79] https://pixabay.com/en/beans-leguminous-plants-legumes-260210/
[80] http://ecommons.cornell.edu/bitstream/1813/3618/2/Weed+Control+for+the+Home+Vegetable+Garden.pdf
[81] https://nassgeodata.gmu.edu/CropScape/
[82] http://soilgrids.org/#/?layer=geonode:taxnwrb_250m (link is external)
[83] http://passel.unl.edu/pages/informationmodule.php?idinformationmodule=1130447032&amp;topicorder=6&amp;maxto=16&amp;minto=1
[84] http://www.arcgis.com/home/webmap/viewer.html?useExisting=1
[85] http://www.prism.oregonstate.edu/normals/
[86] https://planthardiness.ars.usda.gov/PHZMWeb/
[87] https://www.e-education.psu.edu/geog3/sites/www.e-education.psu.edu.geog3/files/Mod6/Module%206.1%20Formative%20Assessment.docx
[88] https://www.usbg.gov/sites/default/files/images/--how_plants_work-_student_discovery_journal-are_plants_like_us.pdf
[89] https://extension.psu.edu/seasonal-classification-of-vegetables
[90] https://commons.wikimedia.org/wiki/File:Tomato_leaf_stomate_1-color.jpg
[91] http://content.ces.ncsu.edu/extension-gardener-handbook/3-botany#section_heading_6937
[92] http://passel.unl.edu/pages/informationmodule.php?idinformationmodule=1092853841&amp;topicorder=5
[93] http://passel.unl.edu/pages/informationmodule.php?idinformationmodule=1092853841&amp;amp;amp;topicorder=5
[94] https://askabiologist.asu.edu/cam-plants
[95] https://www.nature.com/scitable/knowledge/library/effects-of-rising-atmospheric-concentrations-of-carbon-13254108
[96] https://nca2014.globalchange.gov/report/sectors/agriculture
[97] https://www.ers.usda.gov/topics/crops/corn-and-other-feedgrains/
[98] https://www.youtube.com/watch?v=ZitcSUdqmhM
[99] http://faostat3.fao.org/browse/rankings/commodities_by_regions/E
[100] https://www.e-education.psu.edu/geog3/sites/www.e-education.psu.edu.geog3/files/Mod6/FAO_top%2020CommodityChanges_Key.xlsx
[101] https://www.e-education.psu.edu/geog3/sites/www.e-education.psu.edu.geog3/files/Mod6/Summative%20Assessment%206.2%20RevisedSumm2017.docx
[102] https://www.e-education.psu.edu/geog3/node/538
[103] https://www.e-education.psu.edu/geog3/sites/www.e-education.psu.edu.geog3/files/Mod4/Stage2_worksheet_v2.docx