AbstractThe rates of degradation of simazine and linuron were measured in soil from plots not treated previously with these herbicides. Degradation of both compounds followed first‐order kinetics and soil temperature and soil moisture content had a marked effect on the rate of loss. With linuron, half‐lives increased from 36 to 106 days with a reduction in temperature from 30° to 5°C at 4 soil moisture, and from 29 to 83 days at 12 soil moisture. Similar temperature changes increased the half‐life of simazine from 29 to 209 days and from 16 to 125 days at soil moisture contents of 4 and 12 respectively. A computer program which has been developed for simulation of herbicide persistence was used in conjunction with the laboratory data and the relevant meteorological records for the years 1964 to 1968 in order to test the model against previously published field persistence data for the two herbicides. The results with simazine showed a close correspondence between observed and predicted residue levels but those for linuron, particularly in uncropped plots, were satisfactory for limited perio
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