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Diffusion of a Biodegradable Pesticide: II. As Affected by Microbial Decomposition¹

ABSTRACT

The diffusion of parathion (O,O-diethyl O-p-nitrophenyl phosphorothioate) was investigated and a model which involves treating microbial activity in the diffusion system was developed. The kinetics of parathion degradation during 7 days of incubation at three moisture content ( = 0.14, 0.24, and 0.34) was determined and compared with the calculated values. It was found that the rate of parathion decomposition rises with increasing water content. The amount of parathion decomposed at any time increased with the increase in the initial concentration. The calculated course of decomposition of parathion fits the experimental results obtained over 7 days, which was the longest period in the diffusion. The experimental results and predicted distribution of parathion and of the metabolite formed (diethylthiophosphate) are presented as affected by time and soil moisture content. The proposed model—which considers the rate of decomposition at any distance and time, and depends on the local concentration of parathion and on the microbial activity—satisfactorily fits the experimental results.

¹ Contribution from Department of Agricultural Science, University of Oxford, Oxford, U.K.

² Research Associate, Reader in Soil Science, and Visiting Fellow respectively. The permanent address of Z. Gerstl and B. Yaron is: Institute of Soils and Water, Volcani Center, Bet-Dagan, Israel.

Received for publication December 21, 1978. Accepted for publication April 18, 1979.