Modern agriculture depends on chemistry to maximize the yield of crops and animals.Fertilizers, pesticides, and antibiotics are all important applications of chemistry in agriculture.
Fertilizers added to soil provide nutrients for crops, and can be divided into two categories: organic and inorganic. Organic fertilizers are derived from living systems and include animal manure, guano (bird or bat excrement), fish and bone meal, and compost. Organic fertilizers are decomposed by microorganisms in the soil to release their nutrients. These nutrients are then taken up by the plants. Inorganic or chemical fertilizers are less complicated and in most cases more highly concentrated. They can be formulated to provide the correct balance of nutrients for the specific crop that is being grown. Inorganic fertilizers contain higher concentrations of chemicals that may be in short supply in the soil. The major or macro-nutrients in inorganic fertilizers are nitrogen, phosphorous, and potassium. These fertilizers my also provide other micro-nutrients in much smaller quantities.
The amount of land available for agricultural use has decreased as cities have expanded, but the use of fertilizers to increase crop yields has in part offset this loss of agricultural land. The use of fertilizers is not without controversy, however. There are concerns that adding supplements of nitrogen, particularly in the form of inorganic fertilizers, can be detrimental. Some critics believe that adding additional nitrogen to the soil can disrupt the action of nitrogen-fixing bacteria, an important part of the nitrogen cycle. If nitrogen-fixing bacteria in the soil are killed, then less nitrogen is added naturally. As a consequence, more fertilizer must be applied to supply the required amount of nitrogen. Inorganic nitrogen fertilizers are relatively cheap, and they are often added to arable land in excessive amounts. The crop does not assimilate all of this extra nitrogen, but instead the nitrogen can run off the land and enter the water supply. High levels of nitrogen in water can lead to eutrophication, which can in turn trigger algal and bacterial blooms. Algae and bacteria remove oxygen from the water faster than it is replaced by diffusion and photosynthesis, causing some aquatic animals to die from oxygen deprivation. High levels of nitrate in drinking water, which can be due to agricultural runoff, have been implicated in human health problems such as blue baby syndrome (methemoglobinemia).
Pesticides are another important group of agricultural chemicals. They are used to kill undesired organisms--pests--that interfere with agricultural production. Pesticides can be divided into fungicides, herbicides, and insecticides. Fungicides are used to control infestations of fungi, and they are generally made from sulfur compounds or heavy metal compounds. Fungicides are used primarily to control the growth of fungi on seeds. They are also used on mature crops, although fungal infestation are harder to control at this later stage.
Herbicides are weed killers that are used to destroy unwanted plants. Generally herbicides are very selective, because they would be useless for agricultural applications if they killed crops as well as weeds. A general non-selective herbicide can be used to clear all plants from a particular area. However, appropriate treatment must be carried out to remove the herbicide or render it ineffective if that area is to be used for subsequent plant growth. Herbicides can be used to kill weeds that grow among crops and reduce the value of a harvest. They can also be used to kill plants that grow in fields used for grazing by animals, since some plants can be poisonous to livestock or can add unpleasant flavors to the meat or milk obtained from the livestock. Breeding and genetic manipulation are used to introduce herbicide resistance to crops, allowing the use of more broad-spectrum herbicides that can kill more weed species with a single application. Herbicides include a wide range of compounds, such as common salt, sulfates, and ammonium and potassium salts. In the 1940s 2,4-D (2,4 trichlorophenoxyacetic acid) was developed and this herbicide is still widely used today. The use of a related compound, 2,4,5-T (2,4,5 trichlorophenoxyacetic acid), is now controlled because of its potentially harmful effects. 2,4,5-T was a constituent of Agent Orange, a defoliant used during the Vietnam War.
Insecticides are chemicals that are used to kill insect pests, which can spread diseases, eat stored grain, and feed on growing crops. Not all insects are harmful, and certain species of insects are needed to pollinate plants to ensure that they set seed. Many insecticides are non-selective and kill all insects, beneficial as well as harmful. Some insecticides, which are very effective at killing insects, have other problems associated with them. For example, DDT (dichlorodiphenyltrichloroethane) persists in the environment and is concentrated in the food chain. With high levels of exposure, DDT can kill fish and birds by paralyzing their nerve centers. In lower concentrations, it can weaken bird's egg shells and cause sharp declines in reproductive rates. DDT has been banned for all but emergency uses in the United States since 1972, although it is still used in other countries.
Insecticides work in a number of ways. Some are direct poisons (chrysanthemic acids, contact poisons, systemic poisons), while others are attractants or repellents that move the insects to a different location (fumigation acrylonitrile). Some insecticides will attack only at a particular stage of an insect's life cycle.
Antibiotics and growth hormones are used as feed supplements for farm animals. These additives are supplied to keep the animals free from disease and to help them grow to a marketable size as quickly as possible. However, the indiscriminate use of antibiotics can cause problems such as the development of resistant strains of microorganisms or sensitization to the antibiotic among people who eat these animal products. The effects on humans who the meat of animals treated with growth hormones are poorly understood at the present time.
Agricultural chemistry has provided more food at lower cost than ever before. It has also allowed food to be produced in areas that previously were unsuitable for agriculture. Public concerns about the effects of pesticides, antibiotics, and hormones in the food supply chain led to protests during the last decades of the twentieth century. Those protests coincided with an increased demand for organic and so-called green products that are produced without the artificial application of chemicals. Organic products are often more expensive to produce and the increased price is passed on to the consumer. There is, however, research to suggest that appropriate organic techniques can be competitive in cost with typical chemical agriculture and that Third World countries would benefit greatly from many organic practices.
Chemicals have other agricultural uses. For example, sulfur dioxide can be used to keep grain fresh and useable for a longer period of time than untreated grain. Other chemicals can be added to promote the ripening of fruits or the germination of seeds. It is difficult to estimate the monetary value of agricultural chemicals, but many multinational corporations are involved in their manufacture and use.