Agricultural chemicals

The term agricultural chemical refers to any substance involved in the growth or utilization of any plant or animal of economic importance to humans. An agricultural chemical may be a natural product, such as urea, or a synthetic chemical, such as DDT. The agricultural chemicals now in use include fertilizers, pesticides, growth regulators, animal feed supplements, and raw materials for use in chemical processes.

In the broadest sense, agricultural chemicals can be divided into two large categories, those that promote the growth of a plant or animal and those that protect plants or animals. To the first group belong plant fertilizers and animal food supplements, and to the latter group belong pesticides, herbicides, animal vaccines, and antibiotics.

In order to stay healthy and grow normally, crops require a number of nutrients, some in relatively large quantities called macronutrients, and others in relatively small quantities called micronutrients. Nitrogen , phosphorus , and potassium are considered macronutrients, and boron, calcium, chlorine , copper , iron, magnesium, manganese among others are micronutrients.

Farmers have long understood the importance of replenishing the soil , and they have traditionally done so by natural means, using such materials as manure, dead fish, or compost. Synthetic fertilizers were first available in the early twentieth century, but they became widely used only after World War II. By 1990 farmers in the United States were using about 20 million tons (20.4 million metric tons) of these fertilizers a year.

Synthetic fertilizers are designed to provide either a single nutrient or some combination of nutrients. Examples of single-component or "straight" fertilizers are urea (NH₂CONH₂), which supplies nitrogen, or potassium chloride (KCl), which supplies potassium. The composition of "mixed" fertilizers, those containing more than one nutrient, is indicated by the analysis printed on their container. An 8-10-12 fertilizer , for example, contains 8% nitrogen by weight, 10% phosphorus, and 12% potassium.

Synthetic fertilizers can be designed to release nutrients almost immediately ("quick-acting") or over longer periods of time ("time-release"). They may also contain specific amounts of one or more trace nutrients needed for particular types of crops or soil. Controlling micronutrients is one of the most important problems in fertilizer compounding and use; the presence of low concentrations of some elements can be critical to a plant's health, while higher levels can be toxic to the same plants or to animals that ingest the micronutrient.

Plant growth patterns can also be influenced by direct application of certain chemicals. For example, the gibberellins are a class of compounds that can dramatically affect the rate at which plants grow and fruits and vegetables ripen. They have been used for a variety of purposes ranging from the hastening of root development to the delay of fruit ripening. Delaying ripening is most important for marketing agricultural products because it extends the time a crop can be transported and stored on grocery shelves. Other kinds of chemicals used in the processing, transporting, and storage of fruits and vegetables include those that slow down or speed up ripening (maleic hydrazide, ethylene oxide, potassium permanganate, ethylene, and acetylene are examples), that reduce weight loss (chlorophenoxyacetic acid , for example), retain green color (cycloheximide), and control firmness (ethylene oxide).

The term agricultural chemical is most likely to bring to mind the range of chemicals used to protect plants against competing organisms: pesticides and herbicides. These chemicals disable or kill bacteria, fungi , rodents, worms, snails and slugs, insects, mites, algae, termites, or any other species of plant or animal that feeds upon, competes with, or otherwise interferes with the growth of crops. Such chemicals are named according to the organism against which they are designed to act. Some examples are fungicides (designed to kill fungi), insecticides (used against insects), nematicides (to kill round worms), avicides (to control birds), and herbicides (to combat plants). In 1990, 393 million tons of herbicides, 64 million tons of insecticides, and 8 million tons of other pesticides were used on American farmlands.

The introduction of synthetic pesticides in the years following World War II produced spectacular benefits for farmers. More than 50 major new products appeared between 1947 and 1967, resulting in yield increases in the United States ranging from 400% for corn to 150% for sorghum and 100% for wheat and soybeans. Similar increases in less developed countries , resulting from the use of both synthetic fertilizers and pesticides, eventually became known as the Green Revolution.

By the 1970s, however, the environmental consequences of using synthetic pesticides became obvious. Chemicals were becoming less effective as pests developed resistances to them, and their toxic effects on other organisms had grown more apparent. Farmers were also discovering drawbacks to chemical fertilizers as they found that they had to use larger and larger quantities each year in order to maintain crop yields. One solution to the environmental hazards posed by synthetic pesticides is the use of natural chemicals such as juvenile hormones, sex attractants, and anti-feedant compounds. The development of such natural pest-control materials has, however, been relatively modest; the vast majority of agricultural companies and individual farmers continue to use synthetic chemicals that have served them so well for over a half century.

Chemicals are also used to maintain and protect livestock. At one time, farm animals were fed almost exclusively on readily available natural foods. They grazed on rangelands or were fed hay or other grasses. Today, carefully blended chemical supplements are commonly added to the diet of most farm animals. These supplements have been determined on the basis of extensive studies of the nutrients that contribute to the growth or milk production of cows, sheep, goats, and other types of livestock. A typical animal supplement diet consists of various vitamins, minerals, amino acids, and nonprotein (simple) nitrogen compounds. The precise formulation depends primarily on the species; a vitamin supplement for cattle, for example, tends to include A, D, and E, while swine and poultry diets would also contain Vitamin K, riboflavin, niacin, pantothenic acid, and choline.

A number of chemicals added to animal feed serve no nutritional purpose but provide other benefits. For example, the addition of certain hormones to the feed of dairy cows can significantly increase their output of milk. Genetic engineering is also becoming increasingly important in the modification of crops and livestock. Cows injected with a genetically modified chemical, bovine somatotropin, produce a significantly larger quantity of milk.

It is estimated that infectious diseases cause the death of 15–20 of all farm animals each year. Just as plants are protected from pests by pesticides, so livestock are protected from disease organisms by immunization, antibiotics, and other techniques. Animals are vaccinated against species-specific diseases, and farmers administer antibiotics, sulfonamides, nitrofurans, arsenicals, and other chemicals that protect against disease-causing organisms.

The use of chemicals with livestock can have deleterious effects, just as crop chemicals have. In the 1960s, for example, the hormone diethylstilbestrol (DES) was widely used to stimulate the growth of cattle, but scientists found that detectable residues of the hormone remained in meat sold from the slaughtered animals. DES is now considered a carcinogen , and the U.S. Food and Drug Administration has banned its use in cattle feed since 1979.

[David E. Newton ]

RESOURCES

BOOKS

Benning, L. E. Beneath the Bottom Line: Agricultural Approaches to Reduce Agrichemical Contamination of Groundwater. Washington, DC: Office of Technology Assessment, 1990.

——, and J. H. Montgomery. Agrochemicals Desk Reference: Environmental Data. Boca Raton, FL: Lewis, 1993.

——, and T. E. Waddell. Managing Agricultural Chemicals in the Environment: The Case for a Multimedia Approach. Washington, DC: Conservation Foundation, 1988.

Chemistry and the Food System, A Study by the Committee on Chemistry and Public Affairs of the American Chemical Society. Washington, DC: American Chemical Society, 1980.