Illinois Field Crop Applicators Practice Test

When should a higher labeled rate of a soil applied herbicide be used?

1. When soil texture is high in clay

2. When organic matter is low percentage

3. When the herbicide is mixed with others

4. When crop residue soil cover is low

The correct answer is: When soil texture is high in clay

Using a higher labeled rate of a soil-applied herbicide can be beneficial when soil texture is high in clay. Clay soils often retain water and can bind herbicides more tightly than lighter, sandier soils. This retention can reduce the herbicide's availability to the target weeds, potentially leading to insufficient control. As such, increasing the application rate can enhance the likelihood that the herbicide will be effective in penetrating the soil and reaching the root zone of the weeds. Soil texture plays a significant role in herbicide efficacy due to its influence on factors such as adsorption, leaching, and moisture retention. In the case of clay-heavy soils, the higher application rate compensates for these retention challenges and can improve the overall performance of the herbicide against the intended target.