Effect of annual use of pesticides on soil microorganisms and sugar beet yields

Sugar beet is often cultivated for several years on the same fields, using many pesticides. We have therefore studied the effects of a pesticide programme on soil microorganisms and sugar beet yields in Perniö and Laukaa. The pesticides in use were thiram, hymexazol, dimethoate, phenmedipham and metamitron and, in Laukaa only, alloxidim-Na. Pesticides were used either in the normal doses or at 150 % of the normal dose. The normal doses of pesticide application had a favourable effect on sugar beet yields in both experiments. The sugar yield was higher in plots with the normal pesticide doses than in the control plots. The overdoses increased neither the sugar beet yields nor the sugar yields as compared to the normal plots. The soil microorganisms were affected by pesticides in some but not in all cases. The most sensitive were the ureolytic microorganisms and the dehydrogenase activities.