EFFECT OF CROP PRESENCE ON PERSISTENCE OF ATRAZINE, METRIBUZIN, AND CLOMAZONE IN SURFACE SOIL

Atrazine, metribuzin, and clomazone half-lives averaged over treatment s and seasons were approximately 27, 22, and 55 d, respectively, Cloma zone dissipation was not affected by the presence or absence of a soyb ean crop, Atrazine and metribuzin dissipation was not affected by crop s in 1992, but was more rapid in no-crop plots than in cropped plots i n 1993. The difference may have been the result of higher soil water c ontent with no-crop ( a few weeds present) in 1993 than either corn or soybean. Lower soil moisture may have slowed soil microbial activity, thus suppressing atrazine and metribuzin degradation in the 1993 grow ing season. Few significant correlations were found between herbicide half-life or herbicide concentration and cocklebur growth, although on e would expect these to be an index of activity.