EFFECT OF SELECTED HERBICIDES ON ACTIVITIES OF MICROORGANISMS IN SOILS

With the exception of EPTC, herbicide treatments showed inhibitory eff ects on bacterial colony counts in a sandy loam soil for the first wee k. Monolinuron and simazine were stimulatory to the growth of fungi in the organic soil after 2 wk. None of the herbicide treatments affecte d nitrification during the first week of incubation. Except the treatm ent of EPTC in organic soil, all herbicides inhibited nitrification af ter 2 wk in both soils. All herbicide treatments stimulated SO4 format ion during the 8-wk period in the sandy loam soil. Simazine and tridip hane also stimulated sulfur oxidation after 4 wk in an organic soil. W ith the exception of EPTC and nitrapyrin, no significant inhibitory ef fect on the amount of biomass-C was observed in the organic soil. A st imulatory effect on denitrification was observed with EPTC for 2 wk an d monolinuron for 1 wk in the sandy loam soil and with simazine and tr idiphane after 2 wk in the organic soil. It is apparent that the indig enous soil microorganisms can tolerate the effects of the chemicals fo r control of soil weeds.