Technical Report 123

General trends in the soil availability of pesticides

To a large extent, the soil availability (determined by adsorption and desorption) of pesticides is determined by the intended use of the molecule. For example, pre-emergence herbicides (that prevent germination of seeds) should be designed such that, when the herbicide is applied to soil, a sufficient quantity of the active ingredient is available to the target organisms (i.e. not predominantly bound to the soil). These compounds usually have lower Kd values. Contact fungicides or insecticides designed to protect without entering the vascular tissue of a crop tend to have higher Kd values. Systemic herbicides, fungicides or insecticides designed to move within the vascular tissue of a plant have again lower Kd values. Pesticides containing cationic charges or basic functionality require higher Kd values

The Kd of pesticides can therefore cover a very wide range of values. The relationship between Kd and soil parameters in addition to the organic matter content can be an important consideration when assessing the environmental behaviour of pesticides. These parameters include:

• the soil pH, the soil clay content as determined mineralogically
• the nature, proportion and location of metal ions within the soil matrix, and the chemical nature of the soil organic matter