Why is a higher rate of soil-applied herbicides needed in heavy clay and organic matter soils?

A higher rate of soil-applied herbicides is necessary in heavy clay and organic matter soils because herbicides tend to bind with the particles present in these types of soil. Clay particles and organic matter have strong adsorption properties, which means that they can hold onto the herbicide molecules and reduce their availability in the soil solution. This binding effect diminishes the effectiveness of the herbicide, requiring a higher application rate to ensure that enough of the active ingredient remains free to take effect on the target weeds.

In contrast, if an herbicide is applied at a standard rate in soil that has a high level of clay or organic matter, much of the herbicide may become immobilized, rendering it less effective in controlling the desired weed species. Therefore, to achieve adequate weed control, a higher concentration of the herbicide must be applied to compensate for the binding action of the soil components.