Herbicide Interactions with Soil Organic Matter and Clay

There have been numerous books written on the subject of soil/herbicide interactions. It is beyond the scope of this lesson to provide an in depth discussion of this topic; however, it is important for the learner to understand a few basic principles about the interaction of herbicides with soil organic matter and clay. It is important to remember that herbicide absorption can occur by any plant tissue that is in contact with the soil solution. These include seeds, roots, and below ground shoots (coleoptiles for germinating grasses and hypocotyls for germinating broadleaf weeds). There are a few herbicides that are absorbed in the vapor phase, but most herbicides are absorbed from the soil solution. The herbicide concentration in the soil solution depends on soil and herbicide characteristics and their interactions. Soils that are high in clay and organic matter will adsorb much higher amounts of herbicide and herbicide concentrations in the soil solution will be lower. The capacity of soils to adsorb herbicides means that manufacturers recommend higher herbicide application rates on soils with high organic matter and clay content. Herbicides that have low water solubility or that are positive ions will also be strongly adsorbed to organic matter and clay particles. Herbicides that are very tightly adsorbed to the soil will not have herbicidal activity because they are not available for plant absorption.

Herbicide concentrations in the soil solution are not constant, but change in response to the amount of water available to the plant. The amount of herbicide available to the plant is said to be in dynamic equilibrium between the soil, soil solution, free air spaces and the plant. The period of time that root absorption can occur is much longer than for foliar absorption. Anytime sufficient soil water is present, underground plant tissue can absorb the herbicide from the soil solution, assuming that the herbicide has not been degraded by soil microbes.