Sorption of selected pesticides by selected model soils, model soil components, and agronomic soils has been assessed. Sorption by the model soil components gives insight into sorption by the composite substrates.
Pesticide adsorption ratios (= amount sorbed/amount applied) for inorganic substrates were independent of pesticide concentration. Adsorption ratios for substrates containing organic matter decreased with increasing pesticide concentration.
Pesticide sorption ratios for inorganic substrates were essentially independent of time: adsorption was wholly independent; desorption was marginally dependent in some cases. Pesticide sorption by substrates containing organic matter increased with time; equilibrium was not reached in 104 min, but extrapolation suggested it was reached before 105 min.
Sorption curves obtained for the agronomic soils are similar to, sometimes indistinguishable from, those obtained for peat-containing model soils.
Sorption values for composite soils can be calculated, approximately, from sorption values for their components. Calculation is based on apportioning a full share for the dominant component (e.g., peat), a full share of residual free pesticide for the intermediate component (e.g., kaolinite), and a full share of remaining free pesticide for the least reactive component (e.g., sand).