Nourishing Plants with Fertilizers

See the Background section of the lesson Plant Nutrient Deficiencies for further information.

Fertilizers and the Environment

Nutrients provide the basic building blocks of life for all organisms. Proper nutrient application, through fertilizer, improves plant growth and crop yield. However, the nutrient cycle is very complex and humanity is still developing an understanding of the scientific details. Because nutrients occur naturally in the environment, fertilizer application augments nature’s processes. Challenges arise when nutrients, through nitrogen-containing fertilizer, are applied improperly. If nutrients are applied at the wrong rate, in the wrong place, from the wrong fertilizer source, or at the wrong time, then the nutrients may be lost to the environment before the plant(s) can take them up and use them for growth. In such cases, nutrients can be lost from the field either through runoff (water) or through gasification and evaporation (air). In the case of runoff, nutrients can be lost to streams, rivers, lakes, and eventually the oceans. While the hydrologic cycle is very complex, excess nutrients in surface waters can promote growth of algae, which in turn can reduce oxygen concentrations in the water and degrade the overall water quality. In the case of gasification and evaporation, nitrogen is broken down by soil bacteria, is converted to N2O, and moves into the atmosphere as a greenhouse gas.

Nutrient Pollution

Nutrients are a natural part of the environment and enter the biosphere from weathering and erosion processes. Nutrients can enter the environment through agriculture, sewage and wastewater treatment plants, coal-burning power plants, storm water runoff, and automobile exhaust. Nutrient sources vary greatly between urban and rural areas. Controlling nutrient loss means identifying its various sources and implementing policies that limit the loss of nutrients to the environment.

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. Environmental Protection Agency (EPA) estimates that almost 20 percent of the nation’s lakes and 30 percent of streams have high levels of nitrogen and phosphorous pollution. This type of nutrient pollution can cause massive overgrowth of algae. These so-called algal blooms also damage water quality. When large populations of algae die and decompose, they deplete the dissolved oxygen in the water. Marine animals that depend on this oxygen either die or leave the area.

Some species of algae emit toxins that can cause rashes, stomach aches and more serious problems for humans. 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 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 (N2O), a powerful greenhouse gas.

Excess nutrients can 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.

Nutrient Pollution Point Sources

Point sources of nutrient pollution can be tied to specific locations. Typical point sources include factories, power plants, and wastewater treatment 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 New York City.

Nutrient Pollution Non point Sources

Non point sources of nutrient pollution are general sources such as agricultural areas, cities, and automobiles (golf courses, lawns, anything without a distinct discharge point). 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. Automobile exhaust is another non point source. This exhaust releases nitrogen into the atmosphere, but it returns to Earth’s 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 best practices are not used. To avoid introducing nutrient pollution, fertilizers must be applied using the right source, rate, time, and place. Increasingly, farmers are adopting nutrient management and precision agriculture measures that minimize the amount of this pollution.

Regulation of Nutrient Pollution

During the past 40 years, antipollution laws have been enacted to reduce the amounts of toxic substances released into our waters. States, territories, and tribes set water-quality standards. 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 Clean Water 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.

Managing Lawn Fertilizers

Growing concern about algae in surface waters has led some local municipalities to begin regulating lawn fertilizers. Areas in Florida, Illinois, Maine, Maryland, Michigan, Minnesota, New Jersey, New York, Vermont, Virginia, Washington, and Wisconsin have enacted ordinances limiting the phosphorus in lawn fertilizers. In Ontario, Canada, the township of Georgian Bay 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 have increased 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 does not. By 2050, the world’s population is expected to number well over 9 billion people. Food production will need to keep pace. If the world’s population used the world’s farmland evenly, 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, although adoption is growing. 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).
• 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.
• Auto-guidance 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 auto-guidance 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 rely 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 were not modified to do so. Of course, genetic scientists are not 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.