Inhibition of soil urease by organophosphorus insecticides

The effect of three organophosphorus insecticides on soil urease was examined. Inhibition of urea hydrolysis, some 60 days after application of 1000 parts/106 of insecticide to a sandy clay loam, approached 40% (accothion) and exceeded 50% in the case of malathion and thimet. Similar inhibitory effects were recorded using a silt loam soil with which 200 parts/106 application also produced inhibition ranging from 14% (accothion) to 23% (thimet) after 10 days. With lower concentrations of insecticide (50 parts/106) inhibition, though again significant, was of a more ephemeral nature. All three insecticides, at a concentration of 1000 parts/10+6, prevented almost any hydrolysis of urea by jack bean urease. Ureolytic microorganisms, isolated from the soils under investigation, were inhibited by the organophosphates to a greater or lesser extent but the development of tolerance was common. It is suggested that the application of insecticides to control soil-borne insect pests may be a factor in determining the efficiency of urea fertilizer mineralization.