Metribuzin sorption by selected model soils, model soil components, and agronomic soils was assessed. Sorption by the model soil components gave insight into sorption by the composite substrates.
Metribuzin adsorption ratios (= amount sorbed/amount applied) for inorganic substrates were independent of metribuzin concentration. Adsorption ratios for substrates containing organic matter decreased with increasing metribuzin concentration.
Metribuzin sorption ratios for inorganic substrates were essentially independent of time; adsorption was wholly independent; desorption was marginally dependent in some cases. Metribuzin 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 were 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 consists of adding products of free metribuzin and sorption values obtained in the order, dominant component (e.g., peat), intermediate component (e.g., kaolinite), and subordinate component (e.g., sand).