Why do some soil-applied herbicides require higher application rates?

Soil adsorption is a key factor influencing the required application rates of certain soil-applied herbicides. When a herbicide is applied to the soil, it can bind to soil particles through adsorption. This process depends on the chemical properties of the herbicide and the characteristics of the soil, such as its texture and organic matter content.

Herbicides that tend to strongly adsorb to soil particles may require higher application rates to ensure that a sufficient concentration remains in the soil solution to effectively control target weeds. If the herbicide binds too tightly to the soil, it can become unavailable for uptake by the plants, necessitating an increase in the applied amount to achieve the desired level of weed control. This principle is critical in agricultural practice, as it impacts the efficacy of the herbicides used to manage unwanted vegetation.

The other options—photodegradation, chemical degradation, and leaching—represent different processes that affect herbicide persistence and efficacy in the environment but are not directly related to the need for higher application rates due to soil interactions.