DEPTH DEPENDENCE OF DIRECT AND INDIRECT PHOTOLYSIS ON SOIL SURFACES

The photolysis depth of direct and indirect photolysis in soils was determined with use of two agrochemicals. The dinitroaniline herbicide flumetralin and a dialkyl thioether organophosphorus insecticide disulfoton were homogeneously applied to four soils and irradiated. Flumetralin undergoes direct photolysis while disulfoton undergoes indirect photolysis by reaction with singlet oxygen. Various depths of treated soils (0.4–4.0 mm) were exposed to natural sunlight and laboratory lighting until no further degradation of chemical was detected. An estimate for the depth of photolysis was determined by multiplying the soil depth by the percent loss of each chemical. Direct photolysis was found to be restricted to the photic depth of soils (0.2–0.4 mm), while the indirect photolysis depth was slightly deeper. Vertical migration of singlet oxygen to depths greater than the depth of light penetration appears likely, although greater chemical movement of the more volatile disulfoton may account, in part, for the enhancement in indirect photolysis. In all cases photolysis was limited to less than 1.0 mm for laboratory irradiations. Irradiation of the soil in sunlight resulted in greater depths of photolysis (up to 2 mm). © 1990, American Chemical Society. All rights reserved.