PERSISTENCE OF 7 PESTICIDES AS INFLUENCED BY SOIL-MOISTURE

Pesticides are often applied in combination, but less-often is soil pe rsistence measured this way. The present field and laboratory study de termined relative persistence of five herbicides and two insecticides, co-applied, as a function of three soil water contents. Losses were m odeled adequately by first-order dissipation, with no significant impr ovement by using a two-compartment model. The order of persistence in a silt loam, at 25% moisture, was carbofuran < cyanazine < metribuzin = alachlor < atrazine < ethoprop < metolachlor (t(1/2) ranged from 7-9 1 days). Carbofuran degradation increased greatly from 12-25% soil moi sture; atrazine was unaffected by 12-35%, whereas the remaining compou nds showed limited increasing loss in wetter soil. Field-based persist ence was more variable, but generally similar to lab rankings.