Bioavailability and bioactivity of sorbed organic chemicals

Abstract

This chapter uses plants to assay the biological concentration of herbicide in the solution phase, and the difference between the amount of herbicide applied and that detected by the plants as a measure of herbicide bound by the soil colloids. Organic matter and montmorillonite contribute to the binding and reduction in bioactivity of weakly basic compounds in soils, with organic matter contributing the greatest under neutral or alkaline pH conditions and montmorillonite contributing greatly under acidic conditions. Increasing soil pH from 6.5 to 7.0 reduced bioactivity of the herbicide due to lower uptake of the anionic species by plant roots. Soil organic matter binds thiocarbamate herbicides more tenaciously than other soil colloids and reduces bioactivity and vapor losses of the chemicals, and thereby extends their longevity in soils. Bioactivity of thiocarbamate pesticides, dinitroaniline herbicides, and halogenated hydrocarbons was most highly and inversely associated with soil organic matter content.