What is a green manure?
A green manure is a crop growth to improve the soil and benefit the subsequent crop. Once the green manure crop is grown, it is usually incorporated into the soil shortly before sowing the next crop. This return to the soil will increase the amount of organic matter in the soil as well as improving soil fertility.
There are numerous benefits that come from growing green manures in crop rotations. However how green manures perform and whether they are successful or not depends on how they are managed and what the ultlimate goal is of growing them.
The information in this blog comes from a great publication from Cotswold Grass Seeds entitled Sort out your Soil, A practical guide to Green manures. It contains specific information on different species of green manures including details on growing them as well as some of the considerations that are needed from incorporating green manures into your cropping plans.
So where do you start? Below are some practical points to consider in managing green manures on-farm.
Spring or autumn are the best times to sow. This ensures that there is sufficient moisture for germination and that the soil temperature is warm enough. If you do need to plant after September, the best option to use is grazing rye as it will establish rapidly and is good at preventing nitrogen leaching losses over the winter. Green manures can be broadcast or drilled, but ensure that they are sown at the correct depth.
Undersowing is an efficient way of ensuring that a green manure will establish quickly after harvesting. Cereal crops can act as a "nurse crop" protecting a green manure crop in its early stages.
Mowing is an essential part of growing most green manures, especially when the crop is young. Early mowing will make a difference between a well established green manure and one with a high weed burden.
Most species can tolerate being topped close to the ground, but care must be taken with some species, especially fenugreek and vetch. It is best to remove cuttings as this encourages Nitrogen fixation in legumes. If cuttings are left, the rotting material will release N which suppresses the N fixation capacity of the plant.
Green manures can be incorporated ready for the following cash crop by rotavating or ploughing. It is easier to incorporate if the crop has been cut with a flail mower first.
For more information on the benefits a green manure can bring to your soil and information on the different plants available, click here to access the Green Manures booklet.
One of the FCCT case studies Iain Tollhurst, uses green manures in his rotations to great effect, not just on soil structure and building fertility but also in the emissions from his farm. Read more here.