In this lesson students will recognize that fertile soil is a limited resource to produce food for a growing population, describe the role fertilizer plays to increase food productivity, distinguish between organic and commercial fertilizers, and recognize how excess nutrients are harmful to the environment. Grades 6-8

non point source: nutrient pollution that results from runoff and enters surface, ground water, and the oceans from widespread and distant activities

nutrient pollution: the presence of excessive amounts of nutrients such as nitrogen and phosphorus in surface water, groundwater, air, and non-agricultural land; stimulate the growth of algae and phytoplankton, which eventually depletes the waters of oxygen and impacts many aquatic organisms

nutrient toxicity: the presence of an excessive amount of a specific nutrient, which is harmful to the organism

point source poluution: nutrient pollution that comes from a specific source that can be identified such as a factory or a wastewater treatment plant

Nourishing Plants with Fertilizers

Additional information can be found in the Background section of the lesson Plant Nutrient Deficiencies.

Fertilizers and the Environment

No one disputes the fact that proper application of organic and commercial fertilizers increases the yield of crop plants. The concern over their use is that plants may be exposed to larger quantities of nutrients than they can absorb, especially when applied improperly. In such cases, the excess nutrients run off the farmers’ fields with the rain and enter rivers, streams, lakes, and oceans, where they are not wanted. Excess nutrients in aquatic environments promote the growth of algae and similar organisms, leading to a general degradation of water quality. They can also enter groundwater and the atmosphere where they can contribute to human health problems and global warming. Some nutrients are a natural part of the environment and enter the biosphere from weathering and erosion processes. Nutrient sources from humans include agriculture, sewage and waste water treatment plants, coal-burning power plants, and automobile exhaust. The relative importance of these pollutants varies greatly between urban and rural areas. Controlling nutrient pollution means identifying its various sources and implementing policies that limit contact between nutrients and the environment.

Nutrient Pollution

As discussed earlier, organisms require essential nutrients to survive, but they must be present in the proper amounts. Either too little or too much can adversely affect health. A similar situation exists with regard to the environment. The U.S. EPA estimates that 12 percent of the nation’s waters are impaired either by nutrients or by sediment, which also may represent nutrient-related impairments such as oxygen depletion. It has been estimated that more than 60 percent of rivers and bays found in coastal states are moderately to severely degraded by nutrient pollution. Nutrient pollution, especially from nitrogen, can lead to explosive growth of aquatic organisms through a process called eutrophication. The resulting blooms of organisms such as phytoplankton and algae reduce the amount of sunlight available to aquatic vegetation. Their metabolism depletes the bottom waters of oxygen,which can suffocate organisms that cannot move away from oxygen-depleted areas. Scientists have shown that the area of oxygen-depleted bottom water is increasing in estuaries and coastal zones worldwide. Excess nitrate in water supplies can cause human health concerns at high concentrations. The most severe acute health effect is methemoglobinemia, often called ‘blue baby’ syndrome. Recent evidence suggests that there is not a simple association between nitrate and blue baby syndrome, rather that nitrate is one of several interrelated factors that lead to methemoglobinemia. The disease is uncommon in the United States because potential exposure to high levels of nitrate is limited to a portion of the population that depends on groundwater wells, which are not regulated by the Environmental Protection Agency (EPA). Public drinking water systems should contain nitrates at a level safe for consumption as nitrates can be removed by water filtration. Nitrogen pollution from cultivated soils, industry and other sources contributes to global warming because a portion is released into the atmosphere as nitrous oxide (N2 O), a powerful greenhouse gas.

These excess nutrients enter the environment through both natural and human-induced mechanisms. Sources of nutrient pollution are classified as being either point sources or non point sources. Point sources typically are factories, power plants, and wastewater treatment plants,whereas non point sources are general sources, such as farms, cities, and automobiles. A major non point source of nutrient pollution is urban development. For example, clearing of land for housing and industry creates sealed surfaces that do not absorb water and increase nutrient-laden runoff. A related non point source of nutrient pollution is the septic systems that have proliferated as the suburbs extend beyond the reach of urban sewer systems. Another non point source is automobile exhaust. Nitrogen is released first into the atmosphere, but returns to the surface with the rain. Although definitive information is hard to come by, it has been estimated that up to 40 percent of the nitrogen entering aquatic environments in some areas can come from nitrogen in the air. Agriculture is also a non point source for nutrient pollution. Use of fertilizers can send excess nutrients into the environment, particularly when they are applied in excess of the plant’s needs or can quickly move into waterways. Increasingly, farmers are adopting nutrient management and precision agriculture measures that limit the amount of this pollution.

Point sources of nutrient pollution can be tied to specific locations. Most such sources come from wastewater treatment facilities and industrial plants. In urban areas,wastewater treatment facilities can be the largest contributors to nutrient pollution. For example, in Long Island Sound off the East Coast, an estimated 60 percent of the nitrogen that enters the water comes from sewage discharge leaving NewYork City. For many estuaries, however, non point sources contribute more to nutrient pollution than wastewater. In the Mississippi River, point sources account for just 10 to 20 percent of nitrogen and 40 percent of phosphorus entering the system.

During the past 40 years, antipollution laws have been enacted to reduce the amounts of toxic substances released into our waters. Water-quality standards are set by states, territories, and tribes. They classify a given water body according to the human uses the water quality will allow—for example, drinking water supply, contact recreation (swimming), and aquatic life support (fishing)—and the scientific criteria to support those uses. The federal CleanWater Act mandates that if a water body is impaired by a pollutant, a total maximum  daily load (TMDL) must be created. Total maximum daily load is a calculation of the maximum amount of a pollutant that a water body can receive and still meet water quality standards, and an allocation of that amount to the pollutant’s sources. A TMDL is the sum of the allowable loads of a single pollutant from all contributing point and non point sources. The calculation must include a margin of safety to ensure that the water body can be used for the purposes the state has designated—such as swimming and fishing. The calculation must also account for seasonal variation in water quality.

Today, scientists and policy makers are working with farmers to develop more-effective and extensive nutrient management strategies. Solving the nutrient pollution problem will involve establishing emission regulations, compliance incentives, and federal oversight of designated water quality uses. 

Managing Lawn Fertilizers

Growing concern about algae in surface waters has led some local municipalities to begin regulating lawn fertilizers. Areas in Florida, Maine, Michigan, Minnesota, Missouri, Washington, and Wisconsin have enacted ordinances limiting the phosphate in lawn fertilizers. In Ontario, Canada, the township of Georgian Bay recently passed a bylaw banning the application of fertilizer. The merit of such legislation is still under debate. However, manufacturers are responding by offering fertilizer grades with lower amounts of phosphate. Will these approaches be effective in improving water quality in our rivers, lakes, and reservoirs? The principles of nutrient management that have been developed for agricultural fertilizers also apply to lawn fertilizers. With soil testing and wise application, such as more frequent applications at lower doses, nutrient losses can be reduced.

Land Use

Perhaps surprisingly, fertilizers can have a positive impact on the environment with regard to land use. Land is a finite resource, and human societies use it for a variety of purposes. We need land for residential living, for industries, for recreation, for wildlife habitats, and of course, for growing food and fiber. Land cultivation worldwide has remained about the same for the past 50 years. Although subsistence farmers in developing countries have brought some additional land into production, land has also been lost to expanding cities in the developed countries. Even so, starting in the 1960s, farmers were able to increase food production about 400 percent. The Green Revolution was made possible largely by three innovations: better crop varieties, use of commercial fertilizers, and better water management practices. The economist Indur Goklany calculated that if we needed to feed today’s population of over 6 billion people using the organic methods in use before the 1960s, it would require devoting 82 percent of Earth’s land to farming.

The United States produces a surplus of food, but the world doesn’t. By 2050, the world’s population is expected to number well over 8 billion people. Food production will need to keep pace. If the world’s farmland were used evenly by the world’s population, then each person would use 1.8 hectares. Instead, each person in North America uses 9.6 hectares and each European uses 5.0 hectares.

Technology and Nutrient Management

Clearly, if we are going to produce adequate food for our growing population, then crop yields will need to further increase. Strategies will have to be developed to meet the challenges of the future. Some farmers are using technology in a variety of ways to increase crop yields. While the utilization of these new technologies is growing, it is not occurring today on most of the nation’s farms. The rest of this section describes some of these technologies.

Geographic information systems (GIS) allow farmers to use map-based information about natural resources, soils,water supplies, variability in crop conditions throughout the year, and crop yields to ensure the that amount of nutrients being used matches crop needs. Even information about the amount of crop residue (which still contains nutrients) left at the end of the year and the amounts of nutrients removed by the crop can be “mapped” and stored in a GIS database. Once this information is gathered into one database, it can be integrated with other GIS databases such as rainfall records (taken from Doppler radar).

The global positioning system (GPS) is critical to the development of GIS databases and is used to identify the locations of equipment and people in the field. GPS is also useful in assessing general crop conditions and for scouting fields for problems such as nutrient deficiencies. GPS can help farmers return to the same field sites when problems are being addressed.

Autoguidance is a feature of mechanized agriculture. It ties together GPS, GIS, and robotics technologies, allowing a driver to sit and watch as the machine does the work. This technology is being used in various types of farm equipment such as tractors, combines, sprayers, and fertilizer applicators. For example, by using autoguidance systems, farmers can ensure that applications of fertilizers are not on overlapping tracks. The best of these systems can apply fertilizer to an accuracy of less than one inch.

Remote sensing uses satellite images of fields to help farmers know what is happening to their crops. The satellite images can be analyzed to detect variability in the reflection of visible, infrared, and other wavelengths of light. Some images show thermal (heat) radiation from the ground below,which helps estimate soil moisture conditions. These images and data, linked with the GIS data mentioned earlier, offer a means of detecting problems developing in the field and comparing successive images over time. The rate of change can be determined to illustrate how a problem is spreading.

Enhanced efficiency fertilizers help reduce nutrient losses and improve nutrient-use efficiency by crops while improving crop yields. These products provide nutrients at levels that more closely match crop demand leaving fewer nutrients exposed to the environment. Slow- and controlled-release fertilizers are designed to deliver extended, consistent supplies of nutrients to the crop. Stabilized nitrogen fertilizers incorporate nitrification inhibitors and nitrogen stabilizers, which extend the time that nitrogen remains in a form available to plants and reduces losses to the environment.

Gene modification technology is another strategy with potential implications for the future. One of the main factors that limit crop growth is the efficiency of nitrogen uptake and usage by the plant. If crop plants can be made to more efficiently use nitrogen, more fertilizer will be converted into biomass. This means less fertilizer will run off into the environment.

The ultimate goal of this research is to give non-legume plants the ability to obtain their own nitrogen from the atmosphere (i.e. to ‘fix’ nitrogen from the atmosphere) and not relying as heavily on added fertilizers. However, giving a corn plant the ability to fix nitrogen would involve adding a large number of genes, not only from nitrogen-fixing bacteria, but also from an appropriate host plant. The prospect of achieving this anytime soon is remote. Scientists have succeeded in helping plants better use nitrogen by increasing the expression of a single gene. For example, plants that highly express the enzyme glutamate dehydrogenase have been shown to grow larger than those that weren’t modified to do so. Of course, genetic scientists aren’t limiting their efforts to nitrogen fixation. A wide variety of crop plants have been engineered to grow faster, tolerate unfavorable environments, resist pests, and have increased nutritional value.

1. Ask your students if they think we have adequate land to grow and produce enough food for a growing population.  Can every acre of farm land be used to grow food crops or raise animals?  Students may picture areas where there is a lot of open space. However, do they realize that not all land is suitable for growing crops?
2. After completing this lesson, students will be able to:
   • recognize that farmland is a finite resource,
   • appreciate that the world’s growing population demands an increase in food productivity,
   • describe the role fertilizer plays in increasing food productivity,
   • distinguish between organic and commercial fertilizers,
   • describe how excess nutrients are harmful to the environment, and
   • identify different sources of nutrient pollution.

Activity 1: The Big Apple

Tip from the field test: This activity uses an apple as a model of Earth. The Apple Land Use Model can be used as an alternative demonstration option. Students discuss the various ways people use land and make predictions about what percentage of Earth’s land is needed to grow our food. After discussing the ways in which land is used (Step 2), you may consider having the students create their own pie charts where they predict the percentages associated with different land uses, especially farming. Later, their predictions can be compared with the actual values revealed by the apple demonstration.

1. Explain to the class that this activity is concerned with how we as a society use land. The amount of land on Earth stays the same, so as the world’s population gets larger, it becomes even more important that we make wise decisions about how it is used.
2. Explain that land is used for many different reasons. Ask, “What are some of the most important uses for land?” Write students’ responses on the board or an overhead transparency. Students’ responses may include the following:
   • Farming
   • Homes
   • Industries or places where we work
   • Pastures or land for livestock.
   • Parks, sports, and recreation.
   • Wildlife habitat (wetlands, mountain ranges, forests, deserts, beaches, and tundra).
     • If one of these uses is not mentioned by a student, ask guiding questions to bring it out. A student may point out that some land such as a desert has no use. Of course, any land that is not being used by humans can be considered a habitat for wildlife and provides a variety of other economic services for people. For example, wetlands help remove nutrient pollution from rivers, lakes and estuaries.
3. Call attention to the apple and the knife. Explain that the apple represents Earth. Ask, “How much of the total Earth’s surface do you think is devoted to farming?” Students’ responses will vary. Some may remember that about 70 percent of the surface is water.
4. Use the knife to cut the apple into 4 equal parts. Set 3 parts aside and hold up 1 part. Explain that the surface of the world is about 70 percent water, so this 1 piece represents that part of the surface that is land. Remind students of the many different uses for this relatively small amount of land.
5. Use the knife to cut the 1/4 piece of apple in half 3 more times, each time discarding 1/2. Finally, hold up 1 of the smallest pieces and explain that it represents 1/32 of the surface of Earth or 1/8 the land where we live. This is the amount of land available for farming. Point out that the skin on this small piece of apple represents the tiny layer of topsoil that we depend on to grow food.
6. Explain that because we put land to so many different uses, the amount devoted to farming has hardly changed during the past 50 years. Scientists are worried about how we will feed the world’s growing population in the next 50 years.

Activity 2: Using Land Wisely

1. Display a transparency of Master 5.1, Newspaper Articles and cover the bottom portion so that only the top article can be read. Ask for a student volunteer to read the article aloud.
2. Explain to students that they will continue in their roles as agricultural experts concerned with increasing crop yields on farms. Ask students to summarize the content of the article.
   • Try to focus the discussion on the world. Most students in the United States do not have direct experience with severe hunger. Help them understand that in addition to human suffering, hunger can also lead to political instability. It is in everyone’s best interest to eliminate world hunger. The article mentions that population growth contributes to the problem of world hunger. Although population growth is an important societal issue, please remind students that the scope of this module is limited to discussions related to agricultural practices. The article also mentions the availability of freshwater and increasing temperatures due to global warming as challenges for growing more food. If they don’t understand why increasing temperatures cause lower crop yields, explain that it takes more energy and water for plants (and people) to maintain themselves at higher temperatures. Using humans as an example, you can point out that marathon records are usually set at cooler temperatures.
3. Now uncover the bottom article and ask for a second volunteer to read it aloud.
   • Once again, ask students to summarize the article. Students should recognize that there are many factors that influence world hunger and that addressing the problem requires the skills of many different types of people including as social scientists, climatologists, ecologists, water management experts, and agricultural experts.
4. Divide the class into groups of 3 students. Explain that their first task is to investigate how land use is expected to affect farming in the future.
5. Pass out to each group a copy of Master 5.2, Population and Land Use Graphs and Master 5.3, Needs of the Future. Instruct groups to use the graphs on Master 5.2, Population and Land Use Graphs to help them perform a calculation on Master 5.3, Needs of the Future about how much farmland will be needed in the year 2050. Give groups 5 to 10 minutes to perform their calculations.
   • The numbers needed to perform the calculation are indicated on the population graph.
   • For an explanation of calculations, see Teacher's Note
6. Ask each group to report the results of their calculations. Write their answers on the board or on an overhead transparency.
   • If any answers are out of the expected range, go through the calculation step by step, identify the mistake, and correct it.
7. Review the land use for the class. If crop yields stay the same over the next 50 years, then an extra 10 billion acres of farmland will need to be set aside and cultivated.
8. Ask the students to remember the different uses of land that they described in Activity 1: The Big Apple, Step 2. Point to the list of land uses on the board or display the transparency where they are listed.
9. Ask, “If billions of acres of extra farmland are needed to feed people, where should it come from?”  “What are you willing to sacrifice?”
   • Students likely will believe that people must have adequate land for the places where they live and work. They may suggest taking the land from parks or wildlife habitats. Some may suggest that if more people became vegetarians, the extra farmland could come from pastures where livestock graze. These questions are not intended to settle the issue. Instead, they are intended to prompt a discussion that helps students see the scope of the problem and to consider some of the difficult decisions that may lie ahead.
10. Explain that in the next activity, they will consider how farming practices can influence land use and crop yields.

Activity 3: Fertilizers and the Future

Teacher note: In this activity, students read about organic and commercial fertilizers (Master 5.4, Thinking about Fertilizers) and nutrient pollution (Master 5.6, Nutrient Pollution). In both masters, the information is a brief introduction to the topics. The information is not meant to be comprehensive. Rather, it is designed to challenge students’ critical-thinking skills.

1. Remind students that in Activity 2: Using Land Wisely they calculated that 10 billion extra acres of farmland would be needed to feed the world’s population in 2050. Ask, “What assumption was made in reaching this conclusion?”
   • Students’ answers will vary. Some may focus on assumptions associated with the rate of population growth. This is a good answer, but you should guide the discussion to remind students that their calculations assumed that the food yields on farms would remain the same during the next 50 years.
2. Ask, “What will be the effect of increasing the amount of food that an acre of farmland can produce?” 
   • Students should realize that if farmland becomes more productive, then fewer acres will be required to meet the world’s food needs.
3. Explain that in their roles as agricultural experts, they are going to make recommendations to the Earth Food Bank about how to farm in the future. Explain to students that when considering the proper use of fertilizer, they want to increase crop yields, while at the same time minimizing harm to the environment. Proper application of fertilizer means the following:
   • Fertilizer is added at the right time. Fertilizers should be applied during that part of the plant’s life cycle when the nutrients are needed.
   • Fertilizer is added at the right place. Fertilizers should be applied in a location where the nutrients can be taken up by the plant’s root system. This can also mean not adding fertilizer to land that is too close to waterways.
   • Fertilizer is added at the right rate. Fertilizers should be applied at the rate at which the plant can use the nutrients.
4. Explain that students need to learn more about fertilizers and their effects on the environment.
   • Pass out to half of the groups a copy of Master 5.4, Thinking about Fertilizers and a copy of Master 5.5, Pros and Cons of Different Fertilizers.
   • Pass out to the other groups a copy of Master 5.6, Nutrient Pollution and a copy of Master 5.7, Nutrient Pollution Discussion Questions.
   • Instruct the groups to read the information found on the first handout (either Master 5.4, Thinking about Fertilizers or Master 5.6, Nutrient Pollution) and to discuss within their groups their understanding. Students should relate the ideas of “right time, right place, and right rate” when considering the use of fertilizers and their impacts on the environment.
   • Students should use the second handout (either Master 5.5, Pros and Cons of Different Fertilizers or Master 5.7, Nutrient Pollution Discussion Questions) to record their conclusions.
     • Students reading about fertilizers should be able to identify three or four advantages and disadvantages of each type of fertilizer. Students reading about nutrient pollution should be able to describe how excess nutrients can produce algal blooms that use up oxygen in the water, leading to suffocation of other plants and animals. They should be able to identify wastewater treatment facilities and industrial plants as point sources of nutrient pollution. They should identify agriculture, urban development, septic systems, and the burning of fossil fuels as non point sources of nutrient pollution. Student suggestions for limiting non point sources of nutrient pollution will vary. There is no simple correct answer. Look for logical responses that students can defend using evidence. The idea is to get them thinking about the multiple sources of nutrient pollution and for them to realize that limiting its effects will require a complex set of regulations, incentives, and government oversight.
5. After the groups have completed their tasks, ask for volunteers to read their conclusions.
   • Make a list of the advantages and disadvantages of each type of fertilizer on the board or on an overhead transparency.
   • Discuss answers to the questions about nutrient pollution.
6. Ask, “Why do think that some farmers use organic fertilizers and others use commercial fertilizers?" 
   • Student responses will vary. Try to bring out in the discussion that the farmers in the United States have more options than farmers in poorer countries, who may have no choice and must use organic fertilizers that they produce for themselves. A consequence is that farmers in poorer countries obtain lower crop yields as compared with farmers in the United States. However, farmers in the United States often choose to use organic fertilizers for a variety of other reasons.

Teacher note: Try to avoid getting bogged down in debating whether or not food that is organically grown is safer or tastes better than food grown using commercial fertilizers. This is not the focus of the lesson. Scientific studies have not been able to consistently find taste, health, or safety differences between food grown using the two types of fertilizers.

After conducting these activities, review and summarize the following key concepts:

• Fertile soil with an adequate climate for plant growth is a limited resource.
• Soil and water are natural resources that need to be managed and conserved.
• The use of various fertilizers needs to be used correctly to avoid negative environmental impacts.

Nutrients for Life Foundation

• #SoilScience Reader
• Apple Land Use Model
• Nutrients for Life eLessons
• Phosphate Mining Video
• Potash Mining Video
• Soil Health Education Resources
• Topsoil Tour
• Web Soil Survey