Insecticide residues in soils

Abstract

The most common and persistent insecticide residues in soil are DDT and dieldrin, and then lindane, chlordane, heptachlor, and aldrin in order of decreasing persistence. Organophosphorus insecticides are much shorter-lived, remaining in the soil for months at most. The chemical structure of the insecticide and its resultant intrinsic stability is the most important single factor governing its persistence in soil. The more volatile an insecticide the shorter time it persists in soil and the effectiveness of a soil insecticide tends to be inversely proportional to its water solubility. The larger the dose of insecticide applied to soil the less disappears in terms of percentage of the original application in a given time. The formulation of an insecticide affects its persistence in soil; granules persist longer than emulsions, which in turn persist longer than miscible liquids; wettable powders and dusts disappear most rapidly of all. The type of soil treated with an insecticide greatly influences how long residues persist; for instance, heavy clay soils retain insecticides much longer than lighter sandier ones. The phenomenon of adsorption of insecticides on to soil seems to occur with most insecticides; they may, however, become bound to the soil so tightly that they may be non-toxic to either insects or plants. When an insecticide is applied to the surface of mud it gradually becomes adsorbed into the interior of the mud; this must also occur with soil. Organic matter content seems to be the most important single soil factor influencing how long insecticide residues persist in soil, although the amount of clay in a soil can also be correlated with the persistence of residues. There is evidence that organophosphorus insecticides persist longer in acid soils than in alkaline ones and the amount of mineral ions such as Fe, Al, and Mg present can also affect the adsorption and persistence of insecticides in soil. Climatic factors have a considerable effect on soil insecticide persistence. Increased temperatures accelerate the loss of insecticide by increasing conversion to other compounds, volatilization, and desorption, but coincidentally if the soil becomes dry, the loss of insecticide is retarded. The organochlorine insecticides are very weakly soluble and are seldom leached deeper than plough depth. Soil moisture greatly affects both the adsorption of insecticides and their movement through soil; in dry soils insecticides are tightly adsorbed and have much less insecticidal action. Soil insecticides are lost less rapidly from soils with a cover crop, probably because the air movement: over the soil surface is lessened. It has been demonstrated that soil microorganisms are important in the breakdown of soil insecticides. Insecticide residues left on the soil surface disappear many times faster than if they are cultivated into the soil. Four overlapping phases are suggested to analyse the breakdown curves of soil insecticides. These include, firstly, application losses, then a phase of losses due to volatilization from the soil surface. After cufoivation there is a stage during which the insecticide spreads through the soil and becomes stabilized; during this phase, losses are mainly by leaching, adsorption, and volatilization. In the final phase losses continue by leaching and volatilization, but are mainly by enzymatic degradation, and approximate to a regular exponential curve. The various factors affecting soil insecticide persistence can be arbitrarily placed in decreasing order of importance as follows: a) The chemical structure of the insecticide. b) The type of soil to which the insecticide is applied, especially depending on the organic matter content; amount of rainfall reaching treated soil and the moisture in the soil. c) The microbial population of the treated soil; depth of cultivation of the insecticide; mean temperatures in the treated soil. d) The mineral content and acidity of the treated soil; amount of plant cover over treated soil; formulation and concentration of the insecticide.