Persistence and phytotoxicity of napropamide residues in soil

SUMMARY The time for 50% loss of napropamide following incorporation into a sandy loam soil in the field varied from about 130 days when applied in spring or early summer to over 200 days when applied later in the year. When left on the soil surface, up to 75% of the applied dose was lost during the first 28 days after application in June or July, whereas losses following application in November, December or January represented less than 25% of the amounts applied. These losses following surface application were closely correlated with the amounts of incoming solar radiation. First‐order half‐lives for degradation in nine soils incubated moist at 20°C in the laboratory varied from 72 to 150 days. They were positively correlated with the extent of herbicide adsorption by the soils and with their clay content, and negatively correlated with soil pH. Phytotoxicity to wheat and barley in the same soils in glasshouse experiments was inversely related to their organic matter content and to the extent of herbicide adsorption. Wheat was more tolerant of the herbicide than was barley. The results are discussed in terms of possible carryover problems with napropamide in some crop rotations.