Improved agronomic practices, artificial fertilizers, farm chemicals and improved methods of preserving and transporting food are playing roles in agricultural pollution. Although air and water pollutions have affected agriculture adversely, agricultural methods themselves have contributed to environmental pollution.
Some of the important agricultural pollutants are as follows:
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Farm animal wastes:
Over the century farm animal wastes, such as faecal matter, urine, wastes from slaughter houses, dead animal wastes, have been regarded as important sources of soil fertility. Nevertheless, the presence of farm animal wastes pose serious problems of foul odour and water pollution. Sometimes public health problems arise from their role in transmitting diseases. Animal wastes pose serious problems when they enter the water supplies.
Soil erosion:
Eroded materials affect the fertility of the soil. Strong winds carry soil particles into the air and water and thus produce air and water pollution. When they are carried to the streams by storms or by runoff water they settle at the bottom and destroy irrigation systems. Sediments can clog gills of fishes. It interferes with the industrial uses of water and leads to extra- expenses for purification of water. It can reduce the dissolve O2 level and thereby it adversely affects aquatic life. It affects the recreational values of lakes, reservoirs and streams. Control of soil erosion consequently can control the environmental pollution.
Plant residues:
Plant residues from crop fields and orchards contribute much to environmental pollution. They carry plant diseases and pests. When they are burnt they emit smoke and hydrocarbons. Since both the situations correspond to an unfavorable alteration of environment, agricultural burning is important pollution problem.
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Burning of plant residues gives:
8.3% CO,
8.5% particulate matter,
5.3% hydrocarbons,
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1.5% nitrogen oxides.
Much of these burnings is for sanitation purposes since plant debris can carry diseases or pests for succeeding crops. Agricultural burnings and the resulting air pollution are likely to continue until better methods of controlling the plant diseases are available.
Agricultural chemicals:
In recent years there has been increased use of agricultural chemicals, notably pesticides and fertilizers. Pesticides is a general term applied to the substances that kill the disease pests. These include insecticides, fungicides, nematicides, rodenticides and herbicides or weedicides. Thus, agricultural chemicals have no doubt increased crop yields but they have contributed air and water pollution to a significant level.
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They have many adverse effects on fishes, wild life and sometimes human beings. Excessive application of fertilizers may create problems since fertilizers can be transported to ground water by leaching or to waterways by natural drainage and surface runoff. Nitrates in drinking water can cause methaemoglobinemia in babies (blue bodies), because in infant’s stomach the NO3 is converted into NO2 which when reaches the blood reacts with haemoglobin to form methaemoglobin. Phosphorus is often implicated in algal blooms in waterways.
A. Insecticides:
Farmers and gardeners use pesticides to avoid the damage of crops by pests. Insecticides have come in use from the last two decades. Mitter discovered DDT (Dichlorodiphenyl trichloroethane) which is the first chlorinated hydrocarbon to be used as pesticide.
Later on other pesticides were discovered. Insecticides are classified as follows:
(i) Stomach poisons
(ii) Contact poison, and
(iii) Fumigants.
(i) Stomach poisons:
Stomach poisons are arsenic and fluorine compounds; lead arsenate, inorganic fluorine compounds, such as cryollite, sodium fluoride, mercury compounds (HgCl, HgCl2), boron compounds (borax and boric acid), chlorinated hydrocarbons, Organophosphates, carbamates, and certain botanical compounds as pyrethrum (powdered flowers of Chrysanthemum), nicotine. Organophosphorus compounds are most toxic insecticides that affect insects and mammals. Many of them (parathion, thimet, phosdrin, paraxon) are supertoxic and are human poisons giving many symptoms as nausea, vomiting, diarrhea, sweating, salivation and muscular tremors.
(ii) Contact poison insecticides:
Contact poison insecticides are chlorinated hydrocarbons and their metabolites, e.g., DDD, TDE, BHC, DDT which have primary effects on central nervous system. In the environment DDT is degraded into different forms. Chlorinated hydrocarbons are known to be present in the body of fatty human beings, the main source being food supply. Studies have shown that daily content of DDT in meal is about .04 to .05 mg or more. Chlorinated Hydrocarbons persist in the environment for long time. They not only affect large number of insect species but also they are responsible for undesirable side effects.
(iii) Fumigants:
There are gases which are used to kill pests of stored products or nursery stock, e.g., methyl bromide, carbon disulphide, carbon tetrachloride, nicotine, naphthalene, phosphine and other compounds.
B. Fungicides:
These are used to kill fungal pathogens of plants. Sulphur, organic mercuric compounds, formaldehyde, copper sulphate and some others are commonly used.
C. Herbicides:
These are chemicals which kill herbs.
They are of two types:
(i) Non-selective herbicides:
Non-selective herbicides which kill all the plant. The examples are sodium arsenate, sodium chlorate, H2SO4.
(ii) Selective herbicides:
Selective herbicides which kill only certain types of plants. Most commonly used herbicides are 2, 4-D and 2, 4, 5-T and their esters and growth regulating substances. Extensive use of herbicides like ‘picloram’ and cacodylic acid in South Vietnam by the U.S. army has changed the entire ecology of about one-third land area. They caused defoliation of forest trees and destroyed crops and fauna to a great extent. It is horrible act to play with ecosystem and upset the balance of nature by polluting or poisoning the environment just on political ideology. 2, 4, 5-T and 2, 4-D are potential herbicides.
Spraying of these herbicides completely destroyed the vegetation and fauna in estuarian areas of Vietnam. Application of defoliants completely destroyed the forest trees which provided thick cover to Vietnam army. The destruction of climax forest has caused problems of increased erosion, leaching of nutrients and migration of animals from the area. Herbicides and insecticides are very potent pollutants and they greatly affect the structure and function of ecosystems. Their indiscriminate use may be suicidal for our own race.
D. Rodenticides:
They are used to kill rodents, rats and mice. Some have direct immediate action, e.g. strychnine, sodium fluoroacetate, P-thallium and ANTV and others are blood coagulants. The demand for pesticides as a plant protection measure has rapidly increased in India since the introduction of high yielding varieties of crops. A large number of pesticides approved for use under the Insecticide Act, 1968 have been found to be effective against various pests either through direct contact or through systemic action. Some are beneficial as fumigants for protection of food grains during storage.
According to a report of the International Development Research Centre. Ottawa, Canada, about 10,000 people die of pesticide poisoning every year in the developing countries. The alarming rise in the number to such deaths is attributed to increasing use of toxic pesticides. There are about 40,000 such chemicals. A number of such toxic chemicals banned in Western countries are dumped in the Third World countries.
In India, the use of pesticides has risen from 2,000 tonnes a year in the fifties to over 80,000 tonnes in 1985-86 and the areas of application cover both agriculture and public health. The cropped area under plant protection coverage increased from 6.4 million hectares in 1960-61 to over 80 million hectares. The demand for pesticides in 1989-90 was estimated to be about 1.20 lakh tonnes. BHC, DDT, and malathion which account for more than 50 per cent of total production are the cheapest and most popular with small farmers. DDT is mostly used by public health department.
Despite restrictions and regulations on pesticide use, India accounts for one-third of pesticide poisoning cases in Third World. Farm labourers employed for spraying operations are the worst affected as they get exposed for longer period by working continuously during the week. Cases of blindness, cancer, stunted growth of farm workers’ children, deformities; diseases of liver and nervous system due to pesticide poisoning have been identified in the cotton growing districts of two states in India.
Studies conducted by the Industrial Toxicological Research Centre (ITRC), Lucknow in collaboration with the Neurology Department of S.J.M. Medical University, Lucknow revealed that one-fifth of the farm workers have damaged eye-sight following muscular degeneration. There were 106 deaths in Kerala from consumption of wheat flour and sugar that got contaminated with folidol due to leakage during shipment from Mumbai to Cochin, which caused myosis, salivation, pain in abdomen, diarrhea and unconsciousness. Earlier, Harijans in the Chikmagalur district of Karnataka were affected with severe paralysis and other fatal diseases after eating crabs from pesticides treated paddy fields.
Organic pesticides are more effective and relatively less hazardous than inorganic ones. Chlorinated compounds are quite stable and persist in the environment for long period. Their accumulation in the tissue lipoids causes acute toxicity affecting central nervous system leading to hypersensitivity, convulsions, paralysis and death through respiratory arrest. Some pesticides of this group are DDT, BHC, aldrin, dieldrin, lindane, methoxychlor, endrin, etc.
Ethylene dichloride, ethylene dibromide, methyl bromide (halogenated compounds) and aluminium phosphide (inorganic) are commonly used for fumigating cereals and pulses. There is a risk of bromide residue in stored grains. It accumulates in human and animal tissues; liver, kidney, heart, spleen and causes degenerative lesions. People engaged in agriculture are exposed to the risk of pesticides and other chemicals. Due to continuous exposure to such pollutants through air, soil and water, the people face many complications ranging from neurological diseases, gastric disease, respiratory diseases and even cancer (Table 13.19).