METRIBUZIN MOVEMENT IN SOIL COLUMNS

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Abstract
Metribuzin (4-amino-6-tert-butyl-3-(methylthio)-as-triazin-5(4H) one) was added to 1 m columns of Norfolk loamy sand and Cecil sandy loam and distributed within the columns or transported through the columns by added water. The water was added in multiples of a 3 cm unit called a wetting event, based on South Carolina [USA] rainfall data and on a geometric time scale suggested by pesticide disappearance rates in agronomic soils. The wetting schedule utilized 1, 2, 4, 8, 16, 32, 64 and 128 wetting events, equated with 1/4, 1/2, 1, 2, 4, 8, 16 and 32 thirty-two day months of South Carolina rainfall. During its column residence, metribuzin was distributed in the form of Gaussian (bell-shaped) curves. After passage of the equivalent of 8 mo. rainfall, metribuzin had moved through the columns and resided in the effluent. Using an input-outgo accounting system and sorption ratios adjusted in accord with actual water:soil ratios, it was possible to predict distribution of metribuzin, approximately, after water addition. During column residence, the predicted distribution assumed the shape of Gaussian curves, passing progressively from truncated segments to positive skewness to normal development to negative skewness and out of the column. This prediction method may be used as a 1st approximation to describe the distribution of any intact pesticide in any soil at any time after its application. For practical utility, the calculations need adjustment based on pesticide decomposition rate.

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