Rapid Inactivation of Glyphosate in the Soil

In greenhouse studies, soil applications of 14 C-methyl-labeled glyphosate [N-(phosphonomethyl)glycine] were not readily absorbed by corn ( Zea mays L. ‘Michigan 400’) and soybean [ Glycine max (L.) Merr. ‘Hark’]. However, glyphosate available to plants in sand culture was absorbed. Wheat ( Triticum aestivum L. ‘Avon’) a sensitive bioassay plant, was used to detect the herbicide. Clay loam and muck soil rapidly inactivated 56 kg/ha of glyphosate. Autoclaving of the soil did not prevent the inactivation of glyphosate. In a sandy clay loam soil, application of 56 kg/ha of glyphosate decreased plant growth with increasing pH. Additions of 98 or 196 kg/ha of phosphate to the soil surface decreased glyphosate inactivation in the soil. It is postulated that initial inactivation of glyphosate in soil is by reversible adsorption to clay and organic matter through the phosphonic acid moiety.