We have shown by means of laboratory experiments with a wide range of soil types that the decomposition rate of picloram (4-amino-3,5,6-trichloropicolinic acid) in soil is dependent on soil temperature and moisture content and to no significant extent on organic matter, sand, silt, clay, pH, or initial concentration. A fractional order rate law (0.8) describes the disappearance rate best. It was not possible to develop a suitably precise equation for prediction of loss rate as affected by the above soil and climatic factors. Application of the Arrhenius equation to the data indicates an activation energy of 5.4 kcal/mol for the decomposition of picloram in soil, thus suggesting that the reaction is biological rather than chemical.

The 6-hydroxy derivative of picloram (4-amino-3,5,6-trichloropicolinic acid) is not on the main pathway of the soil microbial decomposition sequence for picloram. Rather it may be formed in very small amounts in a competing reaction. Decarboxylation of picloram cannot be the initial reaction in this sequence and, in fact, does not occur.

No abstract

6-Chloropicolinic acid is the sole detectable metabolite, other than carbon dioxide, arising from decomposition of 2-chloro-6-(trichloromethyl) pyridine in soil. The pyridine compound is a potent inhibitor of nitrification now in use with ammonium fertilizers. The purpose of this study was to evaluate the relative influence of various soil and climatic factors on rates of degradation of 6-chloropicolinic acid in soil. Experiments with a wide range of soil types (23 soils) demonstrate that the most important factor influencing the decomposition rate of 6-chloropicolinic acid is soil temperature. When temperature is not a variable, the quantity of organic matter (0.9 to 6.9% by weight) and pH (4.8 to 8.1) significantly affect the rate of decomposition, but sand, silt, and clay percentages do not. Moisture content was without apparent effect because the range of values investigated was too narrow. A fractional-order rate law (0.7) describes the disappearance rate best. Application of the Arrhenius equation to the data for the decomposition of 6-chloropicolinic acid in soil indicates an activation energy of 6.57 kcal per mole, suggesting that the chemical is biologically rather than chemically degraded. It was not possible to develop a suitably precise equation for prediction of loss rate as affected by the above soil and climatic factors because undefined biological factors in the soils override the effect of measurable properties of soil and climate.