Any factor which deteriorates the quality, texture and mineral content of the soil or which disturbs the biological balance of the organisms in the soil is referred to as Land or Soil pollutant. Pollution in soil has adverse effect on the plant growth. Pollution in soil is associated with
1.Indiscriminate use of fertilizers
2.Indiscriminate use of pesticides
3.Dumping of large quantities of waste materials
COMMON SOIL POLLUTANTS
1. Indiscriminate Use of Fertilizers
Fertilizers contaminate the soil with impurities which come from the raw materials used for their manufacture. For instance, As, Pb and Cd present in traces in rock phosphate mineral get transferred to superphosphate fertilizers. Since the metals are not degradable, their accumulation in the soil (due to excessive use of phosphate fertilizers) above their toxic levels becomes an indestructible poison for crops. The quantity of vegetables and crops grown on soil which is constantly treated with fertilizers over the years, goes down. For example, the indiscriminate use of NPK fertilizers reduces the protein content of wheat, maize, grams, etc., grown on that soil. The carbohydrate quality of such crops also gets degraded. Excess potassium content in soil decreases vitamin C and carotene content in vegetables and fruits. The vegetables and fruits grown on over-fertilized soil are more prone to attacks by insects and diseases:
2. Indiscriminate Use of Pesticides
Pesticides are substances which are used to kill or block the reproduction processes of unwanted organisms. Most of the pesticides can be put into one of the following three categories:
(a) Insecticides. These are used to control insects and thus help to curb diseases and protect crops. The important insecticides are chlorinated hydrocarbons, DDT, BHC, aldrin malathion, etc. They are stable in the environment and toxic to insects in small amounts, but much less so to humans.
(b) Herbicides. These are used to kill weeds. Some common weed killers which were used earlier were, NaClO3 NaAsO3, etc. However, because of their toxic effect to mammals their use have been restricted. Now-a-day, organic herbicides such as triazines are widely used as herbicides.
(c) Fungicides. These are used to check the growth of fungi. Fungi are the plants without chlorophyll and cannot use solar energy. They grow at the expense of living organisms or they live on decaying matter. Organic compounds of mercury are frequently used as fungicides.
The remnants of the pesticides used on pests may get adsorbed by the soil particles which may contaminate root crops grown in that soil. Through the consumption of these crops, the remnants may enter the human biological systems affecting them adversely. Most of the pesticides are quite stable and their biodegradation may take weeks and even months. Pesticides not only bring toxic effect on human and animals but also decrease the fertility of the soil.
3. Dumping of large Quantities of Waste Materials
Solid waste include domestic refuse and discarded solid materials such as those from commercial, industrial and agricultural operations. They contain increasing amounts of paper, cardboards, plastics, glass, packing material and toxic or otherwise hazardous substances. A large number of heavy metals get deposited to the soils of the surrounding smelting industries. These effluents in the long run pollute the soil because the chemicals present in the waste are absorbed by the soil. This eventually alters the chemical and biological properties of soil.
It results in floods and cause soil erosion. During the past few years quite a vast green land has been converted into deserts. In India, Thar desert spread annually over 12000 hectares of productive land. Roots of grasses are an excellent binding material and keep the soil intact. Overgrazing, over-cropping and improper tilling accelerate the soil erosion.
CONTROL OF SOIL POLLUTION
The following steps have been suggested to control the soil pollution:
1. The use of chemical fertilizers can be reduced by applying bio-fertilizers and manures. Biological methods of pest control can also reduce the use of pesticides and thereby minimise soil pollution.
2. Recycling and recovery of materials appears to be a reasonable solution for reducing soil pollution. Materials such as paper, glass and some kinds of plastics can be recycled. This would decrease the volume of refuse and help in the conservation of natural resources. For example, recovery of one tonne of paper can save 17 trees.
3. Control of land loss can be attempted through restoring forest and grass cover to check soil erosion and floods. Crop rotation or mixed cropping can improve the fertility of the land.
4. Proper methods should be adopted for the disposal of solid wastes. The simplest and most widely used technique of solid waste management is to bury the waste in locations situated away-from residential areas. While adopting this practice, environmental and aesthetic considerations must be taken into consideration before selecting the dumping sites. Acidic and alkaline wastes should be first neutralised, the insoluble material if biodegradable should, be allowed to degrade under controlled conditions before their disposal. Incineration of wastes is expensive, and leaves a huge residue and adds to air pollution. Pyrolysis, a process of combustion in absence of oxygen on the material burned under controlled
atmosphere of oxygen, is an alternative to incineration. The gas and liquid thus obtained can be used as fuels. Pyrolysis of carbonaceous wastes like firewood, coconut, palm waste, corn combs, cashew shell, rice husk, paddy straw and saw dust, yields charcoal along with products like tar, methyl alcohol, acetic acid, acetone and a fuel gas. Dung should be used to produce “gobar gas” and good manure through gobar gas plants.