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Cover crops

Learn about how to grow cover crops, the benefits and the potential they have to capture carbon.

Image source: Cotswolds Seeds

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Growing cover crops

Cover crops are grown primarily to protect or improve soils between periods of regular crop production, for example over the winter (between harvest of one crop in the summer and the planting of the next crop in the following spring). Cover crops may also be grown over the spring/summer where land might otherwise lay empty. Cover crops can be grown and then destroyed or incorporated into the soil, or they may first be grazed by livestock.

 

Increasingly, cover crops are grown as a mix of two or more species. This can include brassicas such as oil/tillage radish or mustard, cereals such as oats or rye, legumes such as vetch or clover, or others such as buckwheat and phacelia. Careful choice of species is important. Costs of establishing and managing cover crops can be quite high, and multi-species mixtures are not always required.

Multi-species winter cover crops are one of the Actions for soils on arable land and horticultural permanent crops under the Sustainable Farming Incentive (2023), with multi-species spring, summer or autumn cover crops a new Action under the Combined Environmental Land Management offer being introduced in 2024.      

Benefits of cover crops

Cover crops and reduced tillage are fundamental to farm strategies for improving soil physical properties and biological activity, especially where organic manures, composts or other soil amendments are not available.

The most reliable benefits from growing a cover crop in arable rotations are likely to occur in the form of improved soil structure, reduced erosion risk, increased water infiltration, and greater nutrient retention/cycling. These should have benefits for resource efficiency, the environment, and system resilience.

Whilst there is the potential for some positive yield responses with appropriate cover crop use over the course of a rotation, this may depend on the species grown, its compatibility with the main crops in the rotation, and the tillage system. For example, NIAB studies have shown a higher probability of a positive effect on wheat yields from a brassica cover crop in the rotation when used in conjunction with shallow non-inversion tillage rather ploughing. They have also shown a risk of a yield reduction from growing a brassica cover crop for oilseed rape crops grown in the same rotation.

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Capturing carbon in cover crops

Cover crops can form a useful component within a soil carbon capture strategy through the gradual build of soil organic matter. The roots and shoots of cover crops also feed bacteria, fungi, earthworms, and other soil organisms, which can contribute to increased soil carbon levels over time. Studies in the US have indicated that cover crops can promote soil aggregation and that this can in turn improve the ability of soils to store carbon for longer.

Whilst there is general agreement that cover crops can aid carbon capture, the magnitude of this is debated, and may vary with soil type, management and climate. Cover crop species, mix and planting practice may also have an effect. Some studies have suggested that, compared to grasses or brassicas, soils under legume cover crops may have more mineral-associated organic matter, which is less easily degraded than particulate organic matter.

Nevertheless, in the US studies have indicated a range of C capture potential from cover cropping of between 0.5 and 2.0 t/ha C p.a. (1.8-7.5 t/ha CO2e p.a.).

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