Herbicide Equilibrium in Soils in Relation to Soil Water Content

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Abstract
The concept of herbicide partition into adsorbed and solution phases in the soil was utilized to predict the effect of soil water content on the concentration of herbicide in solution. Calculated predictions based on measured partition coefficients and water contents were tested experimentally by equilibrating 2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine (atrazine) with soil from three horizons of a latosolic soil at four water contents (45 to 60%). The effects of adsorption and water content were predicted with reasonable accuracy when only that water held at tensions less than 15 bars was considered available for equilibration with the applied herbicide. Only on low adsorption soils will water content variations alter significantly herbicide concentration in the soil solution. The principal effect of soil water content on herbicide phytotoxicity probably is associated with herbicide transport, which is more sensitive to changes in water content than is the concentration of herbicide in soil solution.