The rates of degradation and mineralization of metribuzin [4‐amino‐6‐(1,1‐dimethylethyl)‐3‐(methylthio)‐1,2,4‐triazin‐5(4H)‐one] were determined in the surface and subsurface horizons of a Dundee silty clay loam soil (Aeric Ochraqualf) from the Mississippi Delta. Residual metribuzin and metabolites were measured by HPLC or TLC methods during or after 91‐d incubations of treated soil in the laboratory. Metribuzin degradation was faster in the surface soil than in the subsurface soils. Degradation in the surface and the subsurface horizons were not described by first‐order kinetics. Degradation was a second‐order process in the surface soil, but only a half‐order process in the subsurface horizons. Considerably more mineralization of 14C‐metribuzin occurred in the surface (0‐ to 10‐cm depth) soil, with 15 to 20% evolved as 14CO2 after 91 d, vs. 5% from the subsurface soils. Substantial amounts of 14C‐bound residues were formed in both surface and subsurface horizons. Reduced microbial populations and activity corresponded to the reduced degradation of metribuzin in the subsoils.

Sorption and desorption of metribuzin [4-amino-6-(1,1-dimethylethyl)-3-(methylthio)-1,2,4-triazin-5(4H)-one] were measured in Dundee silty clay loam surface and subsurface soils. Soil samples were taken from a profile that had been divided into six sections from the surface to a depth of 175 cm. Metribuzin sorbed weakly to all soils from this profile with Freundlich distribution constants ranging from 0.78 to 1.34 μmole/kg. Soils from lower depths of the profile having higher pH (>7.0) and clay contents (>35%) sorbed significantly more metribuzin than the lower clay content, higher organic matter surface soils. Stepwise regression of the distribution constants against the variables pH, organic matter, clay content, and sand content showed that clay was the single best predictor, with sorption increasing as clay content increased (r2=0.750). The combination of two variables most related to sorption was clay and pH (r2=0.860, P= 0.15). Organic matter was not one of the primary variables related to sorption. Metribuzin was easily desorbed in all soils with less than 5% of the originally applied metribuzin remaining after three desorption treatments. This would indicate little tendency for irreversible sorption. This study demonstrates that soil properties within a profile determine sorption and, subsequently, movement of metribuzin.

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Retention is one of the key processes affecting the fate of organic chemicals in the soil-water environment. Retention refers to the ability of the soil to hold a pesticide or other organic molecule and to prevent the molecule from moving either within or outside of the soil matrix. As such, retention refers primarily to the adsorption process, but also includes absorption into the soil matrix and soil organisms, both plants and microorganisms. Retention controls, and is subsequently controlled, by chemical and biological transformation processes. Retention strongly influences chemical transport to the atmosphere, groundwater, and surface waters. Not surprisingly, retention is a primary factor influencing the efficacy of soil-applied pesticides. The literature abounds with references on the retention of pesticides in soils (e.g.,

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