effect of the annual use of some pesticides on soil microorganisms, pesticide residues in soil and carrot yields

The study deals with the effect of common, annually‐used pesticides on soil microorganisms, pesticide residues in soil, and carrot (Daucus carota) yields in Central Finland. Linuron residues in carrot roots were also analysed. Thiram+lindane and dimethoate were applied from 1973–1981 at the commercially recommended doses on experimental plots of carrots, linuron was applied at twice the recommended rate from 1973–1979 and at the normal rate thereafter and in addition TCA was applied in 1978. Maleic hydrazide was used in the years 1973–1976, and glyphosate after 1977. The numbers of different soil microorganisms, their activities and the pesticide residues were studied from autumn 1978 to 1981. The pesticide treatments reduced the growth of soil algae but increased the total number of microorganisms and the number of aerobic spore‐forming bacteria. Linuron residues in the soil were 0.9–2.8 mg kg−1 in the growing season and 1.2–1.7 mg kg−1 in the autumn, 3 months after application. The residues of glyphosate in the soil were 0.7 mg kg−1 in the autumn, 41 days after the treatment, and had declined to a level of about 0.2 mg kg−1 by the following summer. In the pesticide‐treated plots the carrot yield was only 20–60% of the yield in the hand‐weeded plots. The herbicide programme controlled most of the annual weeds but not couchgrass Elymus repens and milk sow‐thistle Sonchus arvensis.