Soil constitutes the upper crust of the earth which support land plants and animals. Like air and water, soil also gets polluted. Soil pollution can be defined ‘the addition of substances to the soil, which adversely affect physical, chemical and biological properties of soil and reduces its productivity. The process of soil formation is very slow, therefore the soil may be considered as non-renewable resource. This makes the problem of soil pollution more acute.
Soil pollution is an extremely complicated process. It may occur directly by dumping, and disposal of wastes, application of agro-chemicals or the indirect result of air pollution such; as acid rains. The main soil pollutants are : (i) industrial wastes (ii) pesticides (iii) fertilizers and manures (iv) discarded wastes (v) radioactive (vi) other pollutants.
(i) Industrial wastes : Both solid and liquid wastes of industries are dumped over the soil. The wastes contain a number of toxic chemicals such as mercury, lead, copper, zinc, cadmium, cyanides, thiocynates, chromates, acids, alkalies, organic substances etc. Some toxic chemicals reach by mining operations also.
(ii) Pesticides: Now-a-days a number of chemicals are used to kill insects (insecticides), fungi (fungicides), algal bloom (algacides), rodents
(rodenticides), weeds (weedicides or herbicides) in order to improve agriculture, forestry and horticulture. They are sprayed on plants in the form of fine mist or powder. Most of the pesticides are broad spectrum and effect all types of life. They are, therefore, also called “biocides”. Pesticides reduce the population and number of species of living organisms including micro organisms, thus effect the structure and fertility of soil. Several pesticides or their degradation products are absorbed by plants which in turn may affect the entire food chains and food webs.
(iii) Fertilizers and manures: Chemical fertilizers are added to the soil for increasing the crop yield. The excretory products of people and livestock and digested sewage singe used as manure pollute the soil. The innumerable pathogens contained in these wastes contaminate the soils and vegetable crops and cause serious health hazards for man and domesticated animals. However, biological sources are the minor factors in altering soil composition.
(iv) Discarded materials: A large number of discarded materials are dumped on the soil by man. These include concrete, asphalt, rugs, leather, cans, plastics, glass, discarded food, paper and carcasses. The addition of solid wastes is sometimes called the third pollution.
(v) Radioactive wastes : Radioactive elements from mining and nuclear power plants, find their way into water and then into the soil.
(vi) Other pollutants : Many air pollutants (acid rain) and water pollutants ultimately become part of the soil. The soil also receives some toxic chemicals during weathering of certain rocks.