Expert response from Andrew Kniss
Associate Professor of Weed Ecology & Management, Department of Plant Sciences at the University of Wyoming
Wednesday, 10/02/2013 14:42
The soil half-life of glyphosate is approximately 47 days (with a range of 2 to nearly 200 days depending on soil type and various environmental conditions). But it is not active for a vast majority of that time. In order for glyphosate to be active as a herbicide, it must first (obviously) enter the plant. But glyphosate binds very tightly to soil particles almost immediately upon reaching the soil, and pesticides are not absorbed by plants while they are bound to the soil. Glyphosate is degraded relatively quickly by soil microorganisms, so there is almost never enough available glyphosate in the soil to cause plant injury. So although glyphosate can be detected in the soil for quite some time after application, it has no practical soil activity as a herbicide. We can spray glyphosate to control emerged weeds and plant a new crop on the very same day in most cases without risk of injuring the crop.
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