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This article throws light upon the five possible fates of pesticides. The fates are: 1. Volatilization 2. Leaching 3. Adsorption 4. Chemical Modification 5. Microbial Metabolism.
Fate # 1. Volatilization:
Soil fumigants among insecticides are highly volatile. A few herbicides (e.g., trifluraline) and fungicides (e.g., PCNB) are highly volatile. These pesticides are lost from soil by vaporization.
Fate # 2. Leaching:
Leaching of pesticides depends upon solubility of pesticides and their potential of adsorption with the soil colloids. In general, herbicides are more mobile than either insecticides or fungicides. Weakly adsorbed herbicides are more susceptible to leaching in the soil than those that are more tightly adsorbed.
Fate # 3. Adsorption:
Some pesticides are adsorbed by soil. The major adsorbing components viz., the silicates and organic matter, are both involved in the adsorption reaction. In general, the order of adsorption of various pesticides is : montmorillonite >>illite>> kaolinite. The dominant clay mineral in the soil, therefore, determines the quantum of adsorption.
A fine-textured soil adsorbs more than a coarse-textured soil having the same dominant clay mineral, because of large surface area of the former than the latter. It is often observed that pesticides in the adsorbed state are less liable to biodegradation. That helps accumulation of pesticides in soil and increase health hazards. In another sense, adsorbed pesticides are less liable to be washed out into the water system and pollute it.
Fate # 4. Chemical Modification:
Some pesticides when come in contact with soil, undergo chemical changes independent of soil microorganism. Chemical breakdown (photodecomposition) of pesticides occur due to solar radiation. Other mechanism of chemical changes may be hydrolysis. These chemical modifications help in the degradation of pesticides in the soil.
Fate # 5. Microbial Metabolism:
Soil microorganism is responsible for major chemical degradation of pesticides. By this biochemical degradation process pesticides are removed from soil. Chlorinated hydrocarbon insecticides such as DDT, deldrin, aldrin are slowly broken down in soil. Thus, these insecticides may persist in soil for 2-5 years.
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In contrast, organophosphate insecticides (e.g., malathion, parathion) persist in soil, only for 1-12 weeks because they are rapidly degraded by a variety of organisms. Risk of environmental pollution is highest with the chemicals having greatest persistence.
The range of persistence of common pesticides in the soil is given below:
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