Winter cover crops and green manures

Fields left with bare soils over winter, particularly on light soils, can often leach nitrates into nearby watercourses and suffer from soil erosion.

Much of our aquatic wildlife requires low nutrient, clean water to survive, and this can be greatly affected where farm inputs enter waterways. This is particularly true for wetland plants, which are largely restricted to ditches where other wet features are absent.

What

Winter cover crops are sown to capture and hold soil nutrients such as nitrates, over the winter, helping to prevent them from being washed out of the soil over the winter. Nutrients entering waterbodies reduce the water quality for aquatic wildlife.

Why

Winter cover crops capture the nitrates from the previous crop to prevent it leaching into waterways. In certain circumstances they can also help reduce soil erosion and the movement of pollutants into watercourses.

Cover crops also help to keep the soil biology alive for the following crop, improve organic matter content (particularly in lighter soils) and provide a source of green cover for wildlife as well as the potential for weed control. Before sowing the next crop, cover crops are tilled into the soil as green manure to return nutrients to the soil.

When the cover crop is then tilled into the soil as green manure, it returns nutrients to the soil. Leguminous species also fix nitrogen from the atmosphere, helping increase soil nitrogen levels in the soil.

Cover crops can also provide shelter for wildlife and enhance soil biodiversity by providing green cover which can in turn act as a habitat for invertebrates that feed insect eating birds overwinter.

Some cover crops have also been shown to improve soil conditions for earthworms.

How

If land is to be left fallow after harvest, or the stubbles of the previous crop cannot be retained through the winter or are weed free then a winter cover crop should be sown into a rough seed bed after cultivation (but before 15 September) to prevent nitrates leaching into watercourses.

Plants that are used as green manures include phacelia, vetch, ryegrass, grazing rye, barley, fodder and tillage raddish, linseed, peas and mustard, or a mix of these (avoid sowing mustard if brassicas are part of the rotation as the susceptibility to clubroot will be increased). You should seek advice to choose a seed mix to suit your conditions.

If you are intending your cover crop to serve as an Ecological Focus Area for greening, you will need to ensure that your seed mix and its management meet the scheme requirements relevant to your country.

Sow at seed rate that will provide a dense cover and protect from soil erosion, and do not apply any fertilisers or manures. Maize and other late-harvested crops can be undersown with a grass or clover-based mix to establish a green cover after the crop has been harvested.

Destroy the cover crop as close to sowing the next crop as possible to reduce the potential for nitrate losses.