Fundamentals of Soil Science (1+1)

Types of pollutant

There are six general kind of pollutant which receive attention in the soil environment.
Pesticides:

• The use of pesticides (insecticide, fungicides, herbicides) in India is increasing at the rate of 2-5 % per annum.
• A large portion of the pesticides applied to control pests and weeds find its way in to the soil which acts as residues.
• Inappropriate application of pesticide can lead to off- target contamination due to spray drift and run-off from plants, causing contamination of the soil.
• Soil flora and fauna may be adversely affected due to contamination. Organisms responsible for nitrification and nitrogen fixation are seriously affected by pesticides.

Inorganic pollutants:

• The group of inorganic pollutants such as mercury, cadmium, lead, arsenic, nickel, copper, zinc, molybdenum, manganese, fluorine and boron which have been found in toxic quantities as they move along the food chain.
• The burning of fossil fuels, smelting and other processing techniques release in to the atmosphere, tons of these elements which can adversely affect surrounding vegetation. These ‘aerosol’ dust particles may be carried for miles and later deposited on the vegetation and soil.
• The domestic and industrial sewage and sludges are major source of potentially toxic chemicals.
• A continuous application of sewage waste over several years may result in enrichment of heavy metals that can have harmful environmental effects.
• The inorganic chemical compounds can be prevented and eliminated by eliminating or drastically reducing the soil application of toxins and by reducing recycling of inorganic chemicals through soil and crop management.

Organic wastes:

• Soil have long been used as disposal sinks for organic wastes.
• The pollution potential of organic wastes, urban and rural, has become a national and even international problem.
• The primary disadvantages of disposing of organic wastes in soils include heavy metal contamination and excess nitrate leaching in to ground water.
• Toxicity of heavy metals and nutrients growth of grass or other plants used for ground cover. This in turn lead to a reduced infiltration rate at the site, limiting the soil usefulness for further waste absorption.

Salts

• Contamination of soils with salts is one form of soil pollution primarily agricultural origin.
• Salt accumulation has been a perpetual problem of civilization in arid and semiarid regions.
• Salt accumulate in the soils because more of them move in to the plant rooting zone than move out. This may be due to application of salt-laden irrigation water in poorly drained soils.
• Salts move up from the lower horizons and concentrate in the surface soil layers.
• The salts are found in heavily populated and industrialized areas where water is returned to streams following its domestic or industrial use.
• Some sevage sludge have sufficiently high levels of salts to cause crop plant damage when the sludge is applied.
• The control of salinity depends almost entirely on water, its quality and management.

Since soils are being used as burial sites for low –level radioactive waste, care should be exerted to be certain the soil properties are such as to discourage leaching or significant plant uptake of the chemicals.

Radionuclides:

• Radioactive elements emit radiations which could be gamma rays, beta rays, alpha particles or neutron. There are several sources from which radioactive contamination might be occurring.
• The fallout from testing of nuclear weapons has resulted in worldwide contamination, while waste products and effluents from nuclear reactors have been the principle source of localized contamination.
• The serious nuclear accident at the Chernobyl power plant in 1986 demonstrated the vulnerability of agricultural soils to radioactive contamination from atmospheric fallout plant.

Acid rain :

• Acid precipitation , popularly called acid rain, is apparently due to the oxidation of nitrogen and sulphur containing gases that dissolve in water vapor of the atmosphere to form nitric acid and sulphuric acid.

• These nitrogen and sulphur oxides move into the atmosphere, be converted to inorganic acids, and return to the soil in rain and snow.
• The continued inputs of acid rain at pH 4.0-4.5 would have significant effects on pH of soils, especially those that are weakly buffered.

Strategies for minimizing soil pollution

• The genetical characteristics, plant species and even verities differ in their susceptibility and tolerance. The leafy vegetables and root crops accumulate larger amounts of heavy metals than grain crops. For example spinach can absorb large amounts of Pb and Cd, whereas, wheat grains accumulate very less amount of toxic heavy metals.
• It will be safe to apply wastes containing toxic constituents less than 1/15th of the amount of nitrogen that will become available.
• There must be judicious reduction in the intended application to the soil of the wastes containing toxic compounds
• Growing of crop plants that have tendency to accumulate the pesticide or following soil management practices leading to increased leaching of the pesticides
• For the minimization of pollution due to N can be achieved by
• Optimum use of the ability of crop plant to compete with processes that lead to losses of nitrogen from soil plant system
• Direct reduction in the rate, duration and extent of losses of N to the environment by loss processes themselves
• Use of high analysis fertilize.
• The application of easily decomposable organic matter can help to reduce pesticide level in the soil.
• The prevention of soil contamination by inorganic toxic compounds particularly heavy metals can be achieved by
• Reducing the application of toxins to the soil.
• By managing the soil-crop system in a manner that leads to the prevention of further cycling of toxins
• Heavy phosphate application to soil can also lead to reduced availability of toxic cations
• The removal of Se through phytoremediation if harvests are not fed to human beings or domestic animals.
• The adverse effect of acid rains in the soil can be reduced or alleviated by drastically reducing the emission of sulphur and nitrogen oxides. Second, the effect of acid rain on soil pH can be overcome by adding lime.