Recent organic fertilizer treatments (cow manure, pig slurry, composts, or green manure) simultaneously increase insecticide adsorption onto soil and the insecticide soil persistence, indicating a mechanism of slow release of insecticide into soil by the organic matter. This occurred in sugar beet crops with aldicarb, thiofanox and imidacloprid; also, in leek, cauliflower and brussels sprouts crops with chlorpyrifos and chlorfenvinphos. In contrast, organic fertilizer treatments applied once or repeatedly in the past, have no significant influence on adsorption or persistence of insecticides; the same is observed for the old soil organic matter, when its soil concentrations change in limited ranges.
It was recently observed that organic fertilizer treatments increase soil microbial activity (Rouchaud et al. 1994a). Because the biodegradation of insecticides in soil is mainly due to microbial activity, organic fertilizer treatments should increase the rates of insecticides biodegradation in crop fields. However, we observed the reverse: the recent organic fertilizer treatments slowed insecticides biodegradation during the first main crop period (Rouchaud et al. 1993). At end of crop periods, the rates of insecticides soil biodegradations increased, and their residues became very low and similar in the organic fertilizers treated or untreated plots. The assumption was made that the slowing down of the insecticides soil biodegradations by the recent organic fertilizer treatments-was due to the increased insecticides adsorptions onto the soil organic matter (Honnay 1993). In order to verify experimentally this assumption, in the present work we measured the aldicarb and thiofanox (applied as granulates in the sowing furrow) and imidacloprid (applied as seed Correspondence to: J. Rouchaud dressing) soil half-lives in organic fertilizer sugar beet trials made in 1993 and 1994. Simultaneously, the soil organic carbon contents of each of the organic fertilizers treated and untreated field plots were measured, as well as the insecticides adsorption coefficients on these soils. Moreover, the soil organic carbon content and insecticides adsorption coefficients were measured for the soils of the sugar beet organic fertilizer trials made in 1991 and 1992, and for which the insecticides soil half-lives were previously reported (Rouchaud et al. 1993, 1994b,1994c, 1994d). This work reports the same measurements for the insecticide chlorpyrifos applied on the soil around the plant stem some days after planting cauliflower or Brussels sprouts crops, and for chlorfenvinphos uniformly sprayed onto the soil one day before leek planting.