By James Hoorman, Hoorman Soil Health Services
The 2020 summer was hotter and drier than normal for most farms, so herbicide carryover will be a major issue for planting cover crops. Herbicides degrade based on soil temperature, rainfall, time of application, organic matter, soil type, soil pH, and sunlight. Generally, microbially active soils break down herbicides quickly. Moisture is critical for microbe activity, so drought or dry summers means slower herbicide breakdown. High soil temperatures can also reduce microbial activity and herbicide breakdown. High soil microbial activity occurs between 75-850F but once soil temperatures get above 900F, generally microbial activity declines. On bare soils, the soil temperatures in the top inch may reach 110-1400F on a hot sunny day, greatly reducing microbial activity and herbicide breakdown.
Herbicide application timing also determines herbicide degradation. Herbicides applied in the spring or early summer have a longer time to break down.