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Picloram Degradation in Soils as Influenced by Soil Water Content and Temperature¹

ABSTRACT

A laboratory experiment was designed to determine the effect of temperature and alternating incubations at field capacity and during drying periods on the degradation of picloram (4-amino-3,5,6-trichloropicolinic acid) in five soils. Picloram was added at a rate of 10 ppm, and degradation was measured by ¹⁴CO₂ evolution resulting from the cleavage of the labeled carboxyl carbon. Picloram degraded very little at 5C and increased only slightly up to 25C. Three soils were highest in degradation rates at 30C while two soils were highest at 50C. Picloram degradation rates during 20-day incubation periods at field capacity, interrupted with 16-day drying cycles, varied among soils and decreased after each successive drying cycle at 30 and 50C, except for one soil at 50C. The degradation rate decreased gradually as water content decreased from field capacity (0.33 bar) to 15 bars tension, and ceased after the soils were air dried.

Key Words: pesticides • herbicides • pesticide persistence • microbial degradation

¹ Contribution from the Agric. Res. Serv., USDA, Western Region, P. O. Box E, Ft. Collins, CO 80522, in cooperation with Colorado State Univ. Exp. Sta., Sci. J. Ser. No. 2044.

² Soil Scientist and Chemist, respectively, USDA.

Received for publication May 7, 1975.