An agrochemical is any chemical substance that humans use to help in the management of an agricultural ecosystem. Agrochemicals include fertilizers, liming and acidifying agents (which are designed to change the pH), soil conditioners, insecticides, herbicides, fungicides, and chemicals used in the raising of livestock such as antibiotics and hormones.
The use of agrochemicals is an increasingly prominent aspect of modern agriculture. As farms have become massive in size, the challenges in keeping the crop free of damage and producing a high yield have increased. Hand-tilling weeds, for example, has become impractical in large-scale, modern agriculture. Thus, agrochemicals have become widely used.
The use of agrochemicals has been critically important in increasing the yield of agricultural crops. However, some uses of agrochemicals cause environmental and ecological damage, which detracts significantly from the benefits gained by the use of these materials.
Fertilizers are substances that are added to agricultural lands to alleviate nutrient deficiencies, allowing large increases in the rates of crop growth. More then 200 million tons of fertilizer are used in agriculture each year worldwide. In the United States, the average rate of fertilizer application is about 97.2 lb per acre (approximately 109 kg per hectare), and about 22 million tons (20 million metric tons) are used in total each year.
The most commonly used fertilizers are inorganic compounds of nitrogen (N). Under conditions where agricultural plants have access to sufficient water, their productivity is most often constrained by the supply of available forms of nitrogen, especially nitrate (NO3-), and sometimes ammonium (NH4+). Farmers commonly increase the availability of these inorganic forms of nitrogen by applying suitable fertilizers, such as urea or ammonium nitrate. The rate of fertilization in intensive agricultural systems is commonly several hundred pounds of nitrogen per acre per year.
Phosphorus (P) and potassium (K) are other commonly applied nutrients in agriculture. Most phosphorus fertilizers are manufactured from rock phosphate, and are known as superphosphate and triple-superphosphate. Some other phosphorus fertilizers are made from bone meal or seabird guano. Potassium fertilizers are mostly manufactured from mined potash.
Often, these three macronutrients are applied in a combined formulation that includes nitrogen, in what is known as an N-P-K fertilizer. For example, a 10-10-10 fertilizer would contain materials equivalent to 10% each of nitrogen (N), P2O5 (source of phosphorus [P]), and K2O (source of potassium [K]), while a 4-8-16 fertilizer would contain these nutrients in concentrations of 4%, 8%, and 16%, respectively. The desired ratios of these three nutrients are governed by the qualities of the soil being fertilized, and by the needs of the specific crop.
Sometimes other nutrients must also be supplied to agricultural crops. Sulfur, calcium, or magnesium, for example, are limiting to crop productivity in some places. Rarely, micronutrients such as copper, molybdenum, or zinc must be applied to achieve optimum crop growth.
Liming and acidifying agents
Agricultural soils are commonly too acidic or too alkaline for the optimal growth of many crop species. When this is the case, chemicals may be added to the soil to adjust its pH to a more appropriate range.
Acidic soils are an especially common problem in agriculture. Acidic soil can be caused by various factors, including the removal of acid-neutralizing bases contained in the biomass of harvested crops, the use of certain types of fertilizers, acid rain, the oxidation of sulfide minerals, and the presence of certain types of organic matter in soil. Because soil acidification is such a common occurrence, acid-neutralizing (or liming) materials are among the most important agrochemicals used, in terms of the quantities added to soil each year.
Acidic soils are commonly neutralized by adding calcium-containing minerals, usually calcite (CaCO3) in the form of powdered limestone or crushed oyster or mussel shells. Alternatively, soil acidity may be neutralized using faster-acting lime (Ca[OH]2). The rate of application of acid-neutralizing substances in agriculture can vary greatly, from several hundred pounds per acre per year to more than 1,000 pounds per acre per year. The rates used depend on the acidity of the soil, the rate at which new acidity is generated, and the needs of specific crops.
Much less commonly, soils may be alkaline in reaction, and they may have to be acidified somewhat to bring them into a pH range suitable for the growth of most crops. This problem can be especially common in soils developed from parent materials having large amounts of limestone (CaCO3) or dolomite (CaMg[CO3]2). Soil can be acidified by adding sulfur compounds or by adding certain types of acidic organic matter, such as peat mined from bogs.
Soil conditioners are organic-rich materials that are sometimes added to soils to improve aeration and water-holding capacity, both of which are very important aspects of soil quality. Various materials can be utilized as soil conditioners, including peat, crop residues, livestock manure, sewage sludge, and even shredded newspapers. However, compost is the most desirable of the soil conditioners. Compost contains large quantities of well-humified organic compounds, and also supplies the soil with nutrients in the form of slow-release organic compounds.
Pesticides are agrochemicals that are used to reduce the abundance of pests, that is, organisms that are considered to interfere with some human purpose. Many kinds of pesticides are used in agriculture, but they can be categorized into simple groups on the basis of the sorts of pests that are the targets of the use of these chemicals. Herbicides are used to kill weeds, that is, non-desired plants that interfere with the growth of crops and thereby reduce their yield. Fungicides are used to protect agricultural plants from fungal pathogens, which can sometimes cause complete failure of crops. Insecticides are used to kill insects that defoliate crops, or that feed on stored grains or other agricultural products. Acaricides (or miticides) are used to kill mites, which are pests of crops such as apples, and ticks, which can carry debilitating diseases of livestock. Nematicides are used to kill nematodes, which are parasites of the roots of some crop species. Rodenticides are used to kill rats, mice, gophers, and other rodents that are pests in fields or that eat stored crops. Preservatives are agrochemicals added to processed foods to help prevent spoilage.
Pesticides are chemically diverse substances. Over 1,000 active ingredients are registered as pesticides. However, each specific pesticidal chemical (also known as the “active ingredient”) may be marketed in a variety of formulations, which contain additional substances that act to increase the efficacy of the actual pesticide. These so-called “inert” ingredients of the formulation can include solvents, detergents, emulsifiers, and chemicals that allow the active ingredient to adhere better to foliage.
Pesticides can also be classified according to the similarities of their chemical structures. Inorganic pesticides, for example, are simple compounds of toxic elements such as arsenic, copper, lead, and mercury. Inorganic pesticides were formerly used in large quantities, especially as fungicides. However, they have largely been replaced by various organic (carbon-containing) pesticides.
A few of the commonly used organic pesticides are based on substances that are synthesized naturally by plants as biochemical defenses, and can be extracted and used against pests. For example, is an insecticide based on pyrethrum, which is obtained from a species of chrysanthemum, while rotenone is a rodenticide extracted from a tropical shrub.
Most organic pesticides, however, have been synthesized by chemists. The synthetic organic pesticides include such well-known groups as the chlorinated hydrocarbons (including the insecticide DDT, and the herbicides 2,4-D and 2,4,5-T), organophosphates (such as parathion and malathion), carbamates (for example, carbaryl and carbofuran), and triazine herbicides (such as atrazine and simazine).
A final class of pesticides is based on the action of bacteria, fungi, or viruses that are pathogenic to specific pests and can be applied as a pesticidal formulation. The most commonly used biological insecticide is manufactured using spores of the bacterium Bacillus thuringiensis, also known as Bt. These spores can be mass-produced in laboratory-like factories, and then used to prepare an insecticidal solution. Insecticides containing Bt are mostly used against leaf-eating moths, biting flies, such as blackflies, and mosquitoes. Most other insects are little affected by Bt-based insecticides, so the unintended nontarget effects of their usage are relatively small.
Agrochemicals used for animal husbandry
Contagious diseases of livestock can be a problem in modern agriculture. This is especially true when animals are being reared at a high density, for example, in feed-lots. Various agrochemicals may be used to control infectious diseases and parasites under such conditions. Antibiotics are especially important in this respect. These chemicals may be administered whenever bacterial diseases are diagnosed. However, antibiotics are sometimes administered with the feed, as a prophylactic treatment to prevent the occurrence of infections. Because of the extremely crowded conditions when livestock are reared in “factory farms,” antibiotics must be administered routinely to animals raised under those circumstances. This practice is contentious, as evidence suggests that the development of antibiotic resistant bacteria may be encouraged.
Sometimes, hormones and other animal-growth regulators are used to increase the productivity of livestock. For example, bovine growth hormone is routinely administered in some agricultural systems to increase the growth rates of cows and their milk production. This use is also contentious; critics contend the unclear health effect of the hormone in humans argues against growth hormone supplementation.
Environmental effects of the use of agrochemicals
Many important benefits are achieved by the use of agrochemicals. These are largely associated with increased yields of plant and animal crops, and less spoilage during storage. These benefits are substantial. In combination with genetically improved varieties of crop species, agrochemicals have made important contributions to the successes of the “green revolution.” This has helped to increase the food supply for the rapidly increasing population of humans on Earth.
The use of certain agrochemicals, however, has also been associated with some important environmental and ecological damages. Excessive use of fertilizers, for example, can lead to the contamination of groundwater with nitrate, rendering it unfit for consumption by humans or livestock. Water containing large concentrations of nitrate can poison animals by immobilizing some of the hemoglobin in blood, reducing the ability to transport oxygen. In addition, the run-off of agricultural fertilizer into streams, lakes, and other surface waters can cause an increased productivity of those aquatic ecosystems, a problem known as eutrophication. The ecological effects of eutrophication can include an extensive mortality of fish and other aquatic animals, along with excessive growth of nuisance algae, and an off-taste of drinking water.
The use of pesticides can also result in environmental problems. Pesticides are used in agriculture to reduce the abundance of species of target pests to below a level of acceptable damage, which is economically determined. Unfortunately, during many uses of pesticides in agriculture, the exposure of other organisms, including humans, is not well controlled. This is especially true when entire fields are sprayed, for example, when using application equipment drawn by a tractor, or mounted on an airplane or helicopter. During these sorts of broadcast applications, many non-target organisms are exposed to the pesticide. This occurs on the treated site, and also on nearby off-sites as a result of “drift” of the sprayed agrochemical. These non-target exposures cause many unnecessary poisonings and deaths of organisms that are not agricultural pests.
In addition, there is a widespread, even global contamination of the environment with some types of persistent pesticides, especially with organochlorines such as DDT, dieldrin, and aldrin. This contamination involves the widespread presence of pesticide residues in virtually all wildlife, well water, food, and even in humans. Residues of some of the chemicals used in animal husbandry are also believed by some people to be a problem, for example, when traces of antibiotics and bovine growth hormones occur in consumer products such as meat or milk.
Some of the worst examples of environmental damage caused by pesticides have been associated with the use of relatively persistent chemicals, such as DDT. Most modern usage of pesticides involves chemicals that are less persistent than DDT and related chlorinated hydrocarbons. However, severe damages are still caused by the use of some newer pesticides. In North America, for example, millions of wild birds have been killed each year as a non-target effect of the routine use of carbofuran, an agricultural insecticide. This is a substantial ecological price to pay for the benefits associated with the use of that agrochemical.
The use of some pesticides is also risky for humans. About one million pesticide poisonings occur globally every year, resulting in 20,000 fatalities. About one-half of the human poisonings occur in poorer, less-developed countries, even though these places account for only 20% of the world's use of pesticides. This disproportionate risk is due to greater rates of illiteracy in poorer countries, and to lax enforcement of regulations concerning the use of pesticides. The rate of self-harm with pesticides in these areas is also greater.
There have been a few examples of pesticides causing extensive toxicity to humans. The most famous case occurred at Bhopal, India, in 1984, in the vicinity of a factory that was manufacturing an agricultural insecticide. In that case, there was an accidental release of about 45 tons (40 tonnes) of deadly methyl isocyanate vapor to the atmosphere. This agrochemical-related emission caused the deaths of about 3,000 people, and tens of thousands of serious injuries.
These and other environmental effects of the use of some agrochemicals are unfortunate consequences of the application of these chemical tools to deal with agricultural problems. Researchers are constantly searching for non-chemical ways of dealing with many of these agricultural needs. Much attention is being paid, for example, to developing “organic” methods of enhancing soil fertility and dealing with pests. Unfortunately, economically effective alternatives to most uses of agrochemicals have not yet been discovered. Consequently, modern agricultural industries will continue to rely heavily on the use of agrochemicals to manage their problems of fertility, soil quality, and pests.