Persistence of seven pesticides as influenced by soil moisture

Pesticides are often applied in combination, but less-often is soil persistence measured this way. The present field and laboratory study determined relative persistence of five herbicides and two insecticides, co-applied, as a function of three soil water contents. Losses were modeled adequately by first-order dissipation, with no significant improvement 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 7-91 days). Carbofuran degradation increased greatly from 12-25% soil moisture; atrazine was unaffected by 12-35%, whereas the remaining compounds showed limited increasing loss in wetter soil. Field-based persistence was more variable, but generally similar to lab rankings.