Agricultural chemicals

DEFINITION: Chemicals utilized by the agriculture industry to improve crop yield or the quality of produce

The chemical fertilizers and pesticides used by the agriculture industry have the potential to stay in soil and water for long periods, creating unintended environmental impacts.

To grow, plants require sunshine, water, carbon dioxide from the atmosphere, and nutrients from the soil. Mineral nutrients may be subdivided into macronutrients (calcium, magnesium, sulfur, nitrogen, potassium, and phosphorus) and micronutrients (iron, copper, zinc, boron, manganese, chloride, and molybdenum). Plant growth, and thus crop yields, is reduced if any one of these nutrients is not present in sufficient amounts. Micronutrients are required in small quantities, and deficiencies occur infrequently; therefore, the majority of agricultural fertilizers contain only macronutrients. Magnesium and calcium are utilized in large quantities, but most agricultural soils contain an abundance of these two elements, either derived from parent material or added as lime. Most soils also contain sufficient amounts of sulfur from the weathering of sulfur-containing minerals, the presence of sulfur in other fertilizers, and atmospheric pollutants. The remaining three macronutrients (nitrogen, potassium, and phosphorus) are readily depleted; these are referred to as fertilizer elements, as they must be added to most soils on a regular basis. Mixed fertilizers contain two or more nutrients. For example, a fertilizer labeled 10-10-10 contains 10 percent nitrogen, 10 percent phosphorus, and 10 percent potassium.

Fertilizers and Environmental Concerns

The application of fertilizer to agricultural soil is by no means new. Farmers have been applying manures to improve plant growth for more than four thousand years. For the most part, this practice had little environmental impact. Since the development of chemical fertilizers in the late nineteenth century, however, fertilizer usage has increased tremendously. During the second half of the twentieth century, the amount of fertilizer applied to the soil increased more than 450 percent. While this increase more than doubled the worldwide crop production, it also generated some environmental problems.

The production of fertilizer requires the use of a variety of natural resources, and some people have argued that the increased production of fertilizers has required the use of energy and mineral reserves that could have been used elsewhere. For every crop, there is a point at which the yield may continue to increase with the application of additional nutrients, but the increase will not offset the additional cost of the fertilizer. The economically feasible practice, therefore, is to apply the appropriate amount of fertilizer that produces maximum profit rather than maximum yield. Unfortunately, many farmers still tend to overfertilize, which wastes money and contributes to environmental degradation. Excessive fertilization can result in adverse soil reactions that damage plant roots or produce undesired growth patterns; it can actually decrease yields, as some micronutrients, if applied in excessive amounts, are toxic to plants and will dramatically reduce their growth.

The most serious environmental problem associated with fertilizers is their contribution to water pollution. Excess fertilizer elements, particularly nitrogen and phosphorus, are carried from farm fields and cattle feedlots by water and are eventually deposited in rivers and lakes, where they contribute to the of aquatic ecosystems. High levels of plant nutrients in streams and lakes can result in increased growth of phytoplankton, a condition known as eutrophication. During the summer months, can deplete oxygen levels in lower layers of ponds and lakes. Excess nutrients can also leach through the soil and contaminate underground water supplies. In areas where intense farming occurs, concentrations are often above recommended safe levels. Water that contains excessive amounts of plant nutrients poses health problems if consumed by humans and livestock, and it can be fatal if ingested by newborns.

Pesticides

Pesticides are chemicals designed to kill unwanted organisms that interfere, either directly or indirectly, with human activities. The major types of pesticides in common use are those designed to kill insects (insecticides), nematodes (nematocides), fungi (fungicides), weeds (herbicides), and rodents (rodenticides). Herbicides and insecticides make up the majority of the pesticides applied in the environment.

The application of chemicals to control pests is not new. Humans have been using sulfur and compounds as insecticides for more than two thousand years. Residues of toxic metals such as arsenic, lead, and mercury continue to be accumulated in plants that are grown on soil where these materials have been used. The commercial introduction of Dichloro-diphenyl-trichloroethane (DDT) in 1942 opened the door for the synthesis of a host of synthetic compounds to be used as pesticides.

Chlorinated such as DDT were eventually banned or severely restricted in the United States because of their low biodegradability and in the environment. They were replaced by organophosphates, which biodegrade rapidly but are generally much more toxic to humans and other animals than chlorinated hydrocarbons. In addition, they are water-soluble and, therefore, more likely to contaminate water supplies. Carbamates have also been used in place of the chlorinated hydrocarbons, and while these compounds biodegrade rapidly and are less toxic to humans than organophosphates, they are also less effective in the killing of insects.

A large number of herbicides have also been developed. These chemicals are generally classified as one of three types: contact herbicides, systemic herbicides, and soil sterilants. Contact herbicides kill when they come in contact with the leaf surface. Systemic herbicides circulate throughout the plant after being absorbed and cause abnormal growth. Soil sterilants kill microorganisms necessary for plant growth and act as systemic herbicides.

Pesticides and Environmental Concerns

Approximately fifty-five thousand different pesticides are available in the United States, and Americans apply about 500 million kilograms (1.1 billion pounds) of pesticides each year. Fungicides account for 12 percent of all pesticides used by farmers, while insecticides account for 19 percent and herbicides account for 69 percent. Billions of pounds of pesticides are applied each year throughout the world. While most of these chemicals are applied in developed countries, the amount of pesticides used in underdeveloped countries is rapidly increasing.

There is no doubt that pesticides have had a beneficial impact on the lives of humans by increasing food production and reducing food costs. However, alongside these benefits, the use of pesticides has caused some environmental problems, including the development of resistance. When a new pesticide is first utilized, it is effective in reducing the number of target pests, but within a few years, a number of species develop genetic to the chemical and are no longer controlled with it, necessitating the development of another pesticide to replace the one that is no longer effective. The pests in turn develop resistance to the newer pesticides, and so on. As a result, many synthetic chemicals have been introduced into the environment, yet the problem is as great as it ever was.

Another problem with pesticides is that they do not go away after they have done their job; they can remain in the for varying lengths of time. For example, chlorinated hydrocarbons can persist in the environment for up to fifteen years. This can be beneficial from an economic standpoint because the pesticide has to be applied less frequently, but it can be detrimental from an environmental standpoint. When many pesticides are degraded, many of their breakdown products, which are often toxic to other organisms, may also persist in the environment for long periods of time.

A third problem with pesticides is their tendency to become more concentrated as they move up the food chain. A pesticide may not affect species at the base of the chain, but it may be toxic to organisms that feed at the apex because the of the chemical increases at each higher level of the food chain. Many algal species, for example, can be sprayed with an without any apparent effect to the algae. However, the chemical can be detrimental to organisms such as birds that eat fish that have eaten insects that have fed on the algae.

A fourth problem is broad-spectrum poisoning. Few, if any, chemical pesticides are selective. They kill a wide range of organisms rather than just the target pests. Many insects are beneficial to the environment, and more damage than good may be done by the use of insecticides that kill these organisms along with the harmful insects.

Many pesticides, particularly insecticides, are also toxic to humans. Thousands of people, many of them children, have died as the result of direct to high concentrations of these chemicals. In addition, many workers in pesticide factories have been poisoned through job-related contact with the chemicals. Numerous agricultural laborers, particularly in countries that have no laws in place specifying stringent guidelines for the handling of pesticides, have also died as a result of direct exposure to these chemicals. Some pesticides have been classified as carcinogens by the US Environmental Protection Agency.

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