Illinois Field Crop Applicators Practice Test

Which conditions lead to lower herbicide activity?

1. Rainfall 2-6 hours after application

2. Temperature is hot

3. Soil moisture is poor

4. Relative humidity is high

The correct answer is: Soil moisture is poor

Lower herbicide activity can occur under conditions of poor soil moisture because herbicides often require some level of soil moisture to effectively penetrate and adhere to the target weeds. When soil is dry, the uptake of the herbicide by plant roots or foliage can be significantly reduced, resulting in less effective weed control. Additionally, many herbicides are designed to interact with water or moisture in the soil to activate their chemical processes. Without adequate moisture, these processes are hindered, leading to diminished herbicide efficacy. Other conditions, like rainfall shortly after application could actually enhance some herbicide activities by helping them to penetrate into the soil, although excessive rain may wash away or dilute the herbicides before they can act effectively. Hot temperatures can also have variable effects on herbicide performance, sometimes increasing activity but also increasing the risk of volatility and degradation. High relative humidity generally aids in the absorption of herbicides by plants, making them more effective. Therefore, the presence of poor soil moisture is a key factor in limiting herbicide activity.