Spatial variability in herbicide degradation rates and residues in soil

Abstract

Rates of degradation of simazine and metribuzin were measured under standard conditions in soil samples taken from different areas of a field. First-order half-lives for metribuzin degradation gave a coefficient of variation of 21·1% whereas those for simazine gave a coefficient of variation of less than 7%. Half-lives in soil samples taken from the same plot at different times were also more variable for metribuzin than for simazine. In a field experiment with simazine, there was considerable variation in initial deposition when the herbicide was applied with a commercial boom sprayer, and variability in soil residues increased with time after application. When the herbicide was applied with a knapsack sprayer, initial deposition was more even and variability in soil residues was always less than when applied with the boom sprayer. It is concluded that the principal cause of the variability in soil residues is unevenness in initial application and that interpretation of soil residue measurements must always make due allowance for the large amount of variation which is often encountered.