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Soil Nitrification Retardation by Rice Pesticides¹

ABSTRACT

The effectiveness of selected rice pesticides carbofuran, molinate, propanil, bifenox, and sodium azide in comparison to nitrapyrin and terrazole in retarding soil nitrification was evaluated by determining increases in nitrate-N concentrations and/or decreases in ammonium-nitrogen concentrations. In a vertisol to which 50 ppm ammoium-N had been added and pesticide applied at recommended application rates, all compounds showed evidence of nitrification inhibition during the first 10 days of incubation. After 60 days incubation, only bifenox, sodium azide, terrazole, and nitrapyrin were still retarding nitrification. Similarly in another experiment the pesticides, applied at one-half to 10 times the recommended application rate, were not as effective as nitrapyrin and terrazole in retarding nitrification in a moist alfisol and vertisol; yet, each pesticide exhibited some nitrification inhibition, especially at the excessive application rate. The effectiveness of the individual pesticides in retarding nitrification varied with soil type. These rice pesticides have potential to retard nitrification that should be recognized when studying N transformations in soils or when studying the effect of pesticides on plants grown in soil.

¹ Contribution by the Texas Agricultural Experiment Station at Beaumont.

² Associate Professor, Texas Agric. Exp. Stn., Route 7, Box 999, Beaumont, TX 77706.

Received for publication December 12, 1978. Accepted for publication June 20, 1979.