INFLUENCE OF TEMPERATURE, SOIL-MOISTURE AND SOIL CHARACTERISTICS ON THE PERSISTENCE OF ALACHLOR

Alachlor degradation in soils followed first-order reaction kinetics. Half-lives in moist soil varied from 11.3 to 34.8 days at 25-degrees-C to 95.9 to 279-6 days at 5-degrees-C, and at 15-degrees-C varied from 15.7 to 83.1 days at 5 kPa soil water stress (field capacity) to 82.8 to 281.4 days at 1500 kPa (permanent wilting point). Degradation rates in laboratory incubations with fluctuating temperatures were predicted with reasonable accuracy from the constant-temperature data. The degradation rate and extent of adsorption were lower in subsoils than in soils from the plough layer. Degradation rate was positively correlated with microbial biomass and microbial respiration, and adsorption was positively correlated with soil organic matter content. Persistence of alachlor in field plots was correlated well with variations in weather pattern during the period September 1990 to July 1991, with an effective half-life varying from 20 to 60 days. Persistence in the field plots was predicted accurately by a computer model of herbicide behaviour.