Green manure reduces weeds while improving soil fertility in organic crop rotations

Organic farmers are prohibited from using harsh synthetic herbicides and therefore weed control is often a major challenge. A recent study published in the journal Agriculture, Ecosystems & Environment found that rotating organic crops with an entire year of a green manure crop can dramatically reduce weed seed banks and help control weeds in the long term. Green manure crops are cover crops that are turned into the soil and used as a source of nutrients to increase soil fertility. While the benefits of cover cropping are well known, there is less research on the long-term effects of turning those cover crops into the soil for green manure. This study measured the amount of weed seeds in soil after terminating a green manure crop and then again one, two and three years later. They measured weed seed banks in soil from a five-year organic crop rotation that began with two years of green manure cropping and compared that to a five-year crop rotation without the green manure. The weed seed bank in the field with green manure was 54% lower and the above ground weed biomass was lower every year for the five-year rotation. This study showed that the legacy effects of weed control lasted for at least three years after implementing green manure cropping, which suggests that this can be an effective weed control strategy for organic farmers.


Banner Photo Credit: Vadim Karnakhin