To be effective, preemergence herbicides need to be moved from the soil surface into the soil zone where most weed seeds germinate, typically the upper inch of soil. The majority of farmers rely on rainfall to accomplish herbicide placement within the profile. Most herbicides will require at least a half-inch of rain for incorporation; the specific amount varies with soil type and initial soil moisture.
Iowa State University
Hartzler, Robert G. and Pringnitz, Brent A., "Weed management in a dry spring" (2002). Integrated Crop Management News. 1750.
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