Soil pollution is the alteration in soil due to the removal or addition of substances, which decreases the productivity and quality of plants. Agrochemicals, industrial wastes, mining wastes, faulty sanitation are the causes of soil pollution.
Agrochemicals and their effects
The use of agrochemicals such as pesticides and fertilizers increase the yield of agricultural crops, but their excessive use cause soil pollution.
Pesticides include insecticides, fungicides, nematocides, rodenticides, herbicides, weedicides, biocides and soil fumigants. These substances are useful in killing pests but at the same time it affects other organisms. Pesticides also enter the body of plants and animals interfere with their metabolism.
Following are the constituents of pesticides:
- Inorganic salts such as those of arsenic.
- Organochlorine, organophosphates and other broad spectrum substances such as D.D.T.
- Hormones- narrow spectrum biochemicals such as 2, 4 dichlorophenoxyacetic acid, 2,4,5 trichloro phenoxy acetic acid etc.
Among the three groups the last two groups of pesticides are common causes of soil pollution. Following are some noticeable effects :
1. Biochemicals such as herbicides, weedicides (e.g., 2, 4 dichlorophenoxy acetic acid, 2,4,5 trichlorophenoxy acetic acid), etc., stimulate excessive cell division and produce tumors. These tumors block phloem in roots and kill the plants. 2,4,5 T also produces local deformities.
2. Organochlorines such as DDT are chlorinated hydrocarbons. These are non degradable and therefore, accumulate in the environment. The concentration of such substances increase as they pass from organisms of one trophic level to another, resulting into highest concentration in man. This is termed as biomagnification. DDT makes many aquatic organism unfit for human consumption, causes death of birds and fish, reduces photosynthetic rates of phytoplankton, damages the nervous system in vertebrates and disturbs the balance of reproductive hormones resulting in delay in ovulation.
Biomagnification refers to the tendency of toxic substances to increase in concentration in progressively higher levels of food chain. Many chemical insecticides, such as DDT and chlordane, resist degradation in the environment, and on ingestion, they accumulate in the body fats. For example, farmers spray DDT on their cabbage plant to control cabbage looper caterpillars. Some of the chemical inevitably goes into rivers, streams and lakes. Some amount of this chemical enters aquatic plants. It enters into the herbivorous fishes which consume these aquatic plants. Fish and other animals cannot break down or excrete the toxin and as a result it accumulates in their body fats.
The magnification continues as the carnivorous fish such as pike, consumes the herbivorous fish. The pike stores all the DDT from many smaller fish it eats. Finally when top carnivores such as fish eating birds, eat fish like pike, all the DDT present in the pike tissue accumulates, in the birds body.
In the birds body high DDT concentration interferes with the calcium metabolism. As a result they produce thin-shelled eggs, which can easily break. Consequently, the number of several carnivorous birds such as falcons, hawks, eagles, etc., have dropped significantly.
Excessive use of fertilizers cause soil deterioration through decrease of natural microflora. Leaching down causes pollution of underground water. Salts entering crop plants in excess may prove harmful. For example nitrate rich leaves, fruits and water produce nitrite in alimentary canal that enters blood, combines with haemoglobin reducing oxygen transport. It may prove fatal in infants. Organic farming involves use of biofertilizers, manure pesticides of organic origin, biological control and resistant varieties.
They include scrap, effluents,sludge, fly ash and radioactive wastes. Industrial solid wastes and sludge add a lot of toxic chemicals into soil resulting in soil pollution. Fly ash is fall out from industrial emissions especially thermal plants. It is also rich in toxic chemicals. Radioactive wastes from testing laboratories and other sources also pollute the soil. Important toxic substances present in industrial wastes are cyanides, chromates, acids, alkalies and metals such as mercury, copper, zinc, lead, cadmium, etc.
They include mine dust, slack and slag. Open cast mining (surface dug out to bring out mineral deposit) completely spoil the surrounding soil. Toxic metals and chemicals present in the mining wastes causes soil pollution, destroys vegetation and produce many deformities in animals and human beings.
They include domestic/ kitchen wastes, market wastes, sweepings, wastes from commercial complexes, hospitals, slaughter house livestock /poultry wastes and trash like waste metals (e.g., cans), plastic bottles, polyethylene carry bags. Hospital wastes include vials, plastic and glass bottles, syringes, needles, organic wastes, chemicals and lot of pathogen carriers. Hospital and domestic wastes are the source of variety of pathogens. Municipal wastes are partly degradable and partly non degradable.
Following are the measures to control soil pollution:
It involves safer land use, planned urbanization, controlled developmental activities, safe disposal and management of solid wastes. The management of solid wastes involve collection and categories of wastes, transport to disposal site and disposal of waste. Disposal of wastes consists of recovery and recycling, source reduction, burning and dumping.
1. Recovery and recycling:
The articles which undergo recovery and recycling are tins, cans and other metal wastes, glass, plastic, polyethylene,rags, paper and cardboard. Metal wastes can be melted and purified. Similarly, broken glass forms new glass. Recycling of waste paper and cardboard results in the formation of cardboard. Waste cotton textile form paper. Plastic forms new but somewhat inferior plastic.
2. Source reduction garbage:
Garbage and other organic wastes are useful for the formation of compost, biogas and manure.
Composting: All types of organic waste of a town are used to prepare a manure termed as compost. In composting the sludge obtained after primary treatment of sewage along with other wastes are allowed to decompose in an open space. The compost is ready for use as manure in 4-6 months.
Gobar gas plants: Cow dung and other organic wastes of farmhouses can now be profitably placed in gobar gas plants which not only enrich manure but also provide gas for domestic use.
Burning is combustion in solid waste having organic waste in open places. It, however produces offensive odours and air pollutants. Better methods are incineration and pyrolysis. Incineration is controlled aerobic combustion of wastes inside chambers at temperature of 900-1300 degree centigrade. Incinerators are fitted with scrubbers and electrostatic precipitators to prevent release of smoke and toxic chemicals. Pyrolysis is combustion inside chambers in the absence of oxygen at a temperature of 1650 degree centigrade.
Fly ash is being converted into bricks for construction work. Fly-ash, industrial effluents containing toxic chemicals and hazardous metals can be used as bedding material for road construction.
4. Dumping (land filling):
Dumping is piling of waste on selected low lying land. It is of two types open and sanitary.
Open dumping (open landfill): It is the accumulation of waste on uncovered low lying area. The waste requires burning at intervals to reduce its bulk.
Sanitary dumping (sanitary landfill): The waste is pulverised, compacted and covered over by a layer of earth.