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.
