Significance of soil properties in the adsorption and mobility of the fungicide metalaxyl in vineyard soils

Adsorption and mobility of the fungicide metalaxyl were studied in 16 vineyard soils from the-La Rioja region (Spain), with organic matter (OM) contents in the 0.31-1.37% range, and in 7 natural soils with OM contents in the 3.30-8.24% range. Adsorption isotherms were obtained using the batch equilibrium technique, and mobility was studied by soil thin-layer chromatography (soil-TLC). In all cases, the adsorption isotherms fit the Freundlich equation. The values of the K-f constants were low in the vineyard soils (0.01-0.64) and increased in the natural soils (1.05-2.83). The nf values were in general lower than unity. K-f constants were significantly correlated (p < 0.001) with the OM content when all of the soils were considered. According to the determination coefficient, r(2), OM would account for 88% of the variance in adsorption. When the vineyard soils alone were considered, a significant correlation was seen between K-f and the OM and clay contents; both parameters, varying simultaneously, explain 80% of the variance in adsorption. Study of the mobility of metalaxyl with soil-TLC indicated that in vineyard soils the fungicide has the potential for being highly mobile in 19% of-the soils and mobile in 81% of them. In natural soils, the fungicide has the potential;for being moderately mobile or mobile in 86 and 14% of the soils, respectively. This type of behavior of metalaxyl indicates that in vineyards soils of the La Rioja region (Spain) with low OM contents, where application of the compound is continuous, a leaching of the fungicide from the soil to groundwaters could potentially occur. These results should be borne in mind when metalaxyl is to be used in the soils of this region.