TAPAN KUMAR MAITRA EXPLAINS THE
INFLUENCE OF PESTICIDES ON THE ACTIVITY OF SOIL MICROFLORA AND FAUNA

Pesticides incorporated into the soil may change the composition of its microflora. Organochlorine insecticides in the doses recommended for controlling soil-inhabiting pests do not have a negative influence on the number of soil micro-organisms. Rapidly decomposing organophosphorus insecticides in the recommended doses stimulate the development of separate groups of micro-organisms, and in increased doses first cause suppression, and then stimulation of soil microflora.
A varying sensitivity to the action of organophosphorus insecticides has been noted in soil micro-organisms. When the cellular structure of the micro-organisms is more complicated, an increased sensitivity to these compounds is observed. At the same time, these insecticides act as a variability factor of the micro-organisms involving their morphological, cultural and physiological characters.
Soil fungicides and fumigants, as a rule, have a negative effect on soil microflora. The biological activity of the soil or the intensity of soil respiration (the absorption of 02 and the evolution of C02) may be used as a general parameter showing how pesticides act on soil microflora.
The change in the activity of soil enzymes in definite conditions characterises the effect of pesticides on the microbiological activity in the soil. The effect of pesticides on biological processes in the soil comes to light most clearly only upon a repeated or multifold application of the toxicants.
Herbicides decompose comparatively rapidly in soil and their use in recommended doses as a whole does not affect soil microflora negatively. When herbicides are incorporated into the soil, especially in increased doses, a temporary regrouping occurs in the composition of the microflora.
The action of herbicides on separate groups of micro-organisms manifests itself differently within the limits of each systematic group. After incorporation of the herbicides, a short period of depression of the microflora activity sets in. The latter is restored owing to the appearance of resistant mutant forms or to the formation of enzymes that hydrolise the herbicide.
The nature of herbicide action on soil microflora depends in many respects on the features of the structure and properties of the substances. Herbicides that are derivatives of phenoxycarboxylic acids (2, 4-D, MCPA, 2, 4-DB, MCPB, MCPP) do not have an inhibiting action on soil micro-organisms in their recommended doses. It has been established that phenoxybutyric acids (MCPB, 2, 4-DB) are much more toxic to soil micro-organisms than the phenoxy-acetic analogues corresponding to them.
During the first four weeks after its incorporation, 2,3,6-TBA lowers the activity of some species of soil micro-organisms, inhibiting the process of nitrification. Sodium trichloroacetate (TCA) immediately after introduction may inhibit active soil microflora, while dalapon stimulates its activity. Picloram, even in large doses, does not suppress the development of the main species of soil bacteria and fungi, nor affect the evolution of CO2 by the soil and the hydrolysis of urea. It must be stressed that it is just the indifference of soil microflora to picloram that explains its prolonged toxic action.
Herbicides that are triazine derivatives do not affect the development of most soil micro-organisms in their conventional doses. Simazine and atrazine somewhat stimulate the activity of Azobacter and Clostridium pasterianum without affecting Nitrosomonas, but to some extent inhibit the oxidation of nitrites to nitrates under the action of Nitrobacter. The development of ammonifying, denitrifying micro-organisms and rhizosphere microflora is stimulated under the action of these substances. The inhibiting action of simazine manifested itself in the development of soil algae.
The most sensitive were blue-green algae. Carbamates and thiocarbamates initially suppress the activity of micro-organisms. Isopropyl-N-phenyl carbamate can inhibit the activity of nitrifying bacteria. Urea derivatives in conventional doses do not inhibit the development of soil micro-organisms, but do exhibit a rather strong algicidal action.
The nature and degree of action of pesticides on soil fauna are due to the properties of the substances, their content in the soil, the composition of the fauna and to soil and climatic conditions. In some cases, pesticides stimulate the reproduction of soil fauna, in others they cause its suppression and extermination.
Unstable, rapidly decomposing pesticides are less dangerous to soil fauna. Persistent compounds upon their surplus accumulation are a great danger. Organochlorine insecticides such as HCH, heptachlor and toxa-phene in their usually recommended doses act weakly on or are completely harmless to earthworms, nematodes, and Oligochaeta (Ehchytracidae), but are toxic to soil arthropods (springtails, mites). Organophosphorus insecticides have a short action on soil fauna.

THE WRITER IS ASSOCIATE PROFESSOR, HEAD, DEPARTMENT OF BOTANY, ANANDA MOHAN COLLEGE, KOLKATA, AND ALSO FELLOW, BOTANICAL SOCIETY OF BENGAL, AND CAN BE CONTACTED AT
tapanmaitra59@yahoo.co.in