Diffusion of organophosphorus insecticides in soils

Diffusion of disulfoton and dimethoate in a silt loam soil was studied over a range of concentrations and moisture contents. Apparent diffusion coefficients were calculated from the distribution of insecticide in a column of soil after diffusion from one half of the column to the other for a known time. The distribution was determined by slicing the column into narrow; sections using a specially constructed diffusion cylinder. Diffusion coefficients varied little with concentration for both insecticides, but increased rapidly with increasing moisture content for dimethoate from 3.31 × 10 −8 cm 2 /sec at 10% volumetric moisture content to 1.41 × 10 −6 cm 2 /sec at 43% moisture content. In contrast, for disulfoton which is more volatile, less soluble and more strongly sorbed than dimethoate, diffusion coefficients were smaller (2.83 × 10 −8 cm 2 /sec at 41% moisture content) but did not change much as the soil became drier (2.74 × 10 −8 cm 2 /sec at 8 % moisture content). The influence of partition between solid, solution and vapour phases in the soil and of the geometry of the pathway‐through the soil pores on the apparent diffusion coefficient is discussed. The likely behaviour of other pesticides is considered in the light of these results.