Soil health
Cover crops can deliver many benefits to soil health, including maintaining aggregation of soil particles, feeding soil flora and fauna and aiding drainage (by putting down roots).
One of the core principles of regenerative agriculture is to keep living roots in the ground at all times. Cover crops can help fulfil this role. For example, by plugging the gap between harvest and the establishment of a spring crop.
Improved infiltration and reduced erosion and run-off
Cover crops are grown primarily for the purpose of ‘protecting or improving’ between periods of regular crop production. They can reduce the risk of surface run-off and diffuse pollution by providing soil cover. How much the cover crop does this depends on the seed mixture.
Species with a high root density (influencing soil aggregate stability, such as oats) and high root thickness (influencing drainage pores, such as radish and chicory) tend to perform especially well.
Species that establish rapidly and provide good levels of dense green cover (such as mustards) are also likely to perform well in reducing the direct erosive force of raindrops on the soil surface (interception).
Water infiltration rates provide information on soil structure. Improving soil’s ability to absorb water during intense rainfall events (infiltration) helps reduce the risk of soil erosion and the associated loss of nutrients from land to water.
Cover crops can add organic matter to soils, which can improve the soil’s physical condition, helping to stabilise soil aggregates, reduce compaction, and stimulate microbial activity. These improvements in soil structure help to retain more water within the soil rooting zone for use by crops, and reduces the risk of sub-surface flow and leaching.
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Alleviating compaction
Deeply penetrating roots of some cover crop species can act as a ‘tillage tool’, creating channels that can be used by crop roots, alleviating soil compaction and improving soil structure. However, do not expect rooting action to get rid of compaction pans. These should be removed with targeted sub-soiling and followed by cover crops to keep compaction at bay and retain the structure created by the cultivation.
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(Image: Yorkshire Agricultural Society)
Carbon capture
Typically, plants capture carbon dioxide from the atmosphere during photosynthesis and store it as biomass.
Cover crops can also reduce a farm’s carbon footprint by improving soil biology and structure.
As biomass increases, earthworm numbers often rise. This, along with more roots, increases infiltration rates in the soils. The root exudates also contribute organic carbon to the soil and aggregation.
Earlier drilling has been shown to help increase biomass in some trialled species, including black oats, phacelia and clover.
Healthier soils also capture and retain more carbon, through the accumulation of soil organic matter and increased levels of soil biology.
Early indicators that assess soil biological activity, like respiration and microbial tests, can often demonstrate this potential earlier than some soil carbon benchmarking.
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(Image: AHDB)
Livestock grazing
Well-managed livestock can do a fantastic job of reducing the bulk of a cover crop and start breaking down organic matter (and adding to it, via manure).
Many favour grazing by sheep to terminate cover crops (the ‘golden hoof’ treatment).
Cattle are also used, although their extra weight can increase soil compaction and poaching risks, particularly on heavy soils.
Grazing winter cover crops can lead to some other agronomic issues, such as leaching of nitrogen deposited in urine (especially in adverse weather conditions). Mitigate such risks by having a ‘back-up’ field nearby, to move livestock to, if ground conditions worsen.
The grazing potential of various cover crop species is detailed on the species selection page.
Further information on livestock grazing is on the termination page and the AHDB website.
(Image: AHDB)
Nutrient capture
Over winter cover crops are commonly used to take up nutrients on what would otherwise be bare or stubbled ground. This can help reduce over-winter leaching.
The amount of nitrogen taken up by over-winter cover crops is affected by species and biomass achieved.
Uptake of nitrogen by cover crops sown in late summer/autumn ranges from 30 to 120 kg N/ha before spring.
The nitrogen fixed or captured by the cover crop must be broken down to plant available forms for use by following cash crops. The speed of this process depends on the rate of mineralisation (which is influenced by many factors).
The method and timing of cover crop destruction may affect nitrogen release. Further research is required to understand this better.
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Nitrogen release from Cover Crops (NiCCs) project*
- This project investigated the legacy effect of cover crops on subsequent crops in the rotation (at two trial sites)
- Cover crop mixtures significantly reduced nitrate leaching (compared to over-winter stubble) and spring soil nitrogen supply (SNS) was up to 30kg/ha greater
- The second phase of the project is improving understanding of the fate of this nitrogen and whether it translates into a reduction in fertiliser requirements for following crops
*A collaborative project with ADAS, Affinity Water and Portsmouth Water.
(Image: AHDB)
Biodiversity
Cover crops provide a diverse range of benefits to biodiversity.
Above ground
- Autumn/winter flowering resources for insects that feed on nectar or pollen
- Provide habitat, shelter and protection for insects and animals (particularly during the winter months)
- Add to the diversity of vegetation, which can increase the abundance of natural enemies of pests
Take care to avoid ‘green bridges’ for pests and diseases of the following crop, especially when the cover crop and crop are closely related. For example, many plant species act as a host for aphids. Early termination may be beneficial to manage aphids.
(Image: Yorkshire Agricultural Society/Simon Hill Photography)
Below ground
- Provide a food source for many soil organisms that carry out essential below-ground functions, such as the decomposition of soil organic matter (SOM) and transformation of nutrients into plant available forms
- Provide living plant roots and sugary exudates that serve as habitat and food for beneficial organisms, such as mycorrhizal fungi
- Support wider soil microbial communities and increase overall microbial biomass
- Increase macro fauna, particularly earthworms, which contribute to overall soil health (the effects depend on other factors, such as the tillage approach). More than 8 earthworms per soil pit represent an ‘active’ population for arable soils
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(Image: AHDB)
Biofumigation
Some ‘biofumigant’ cover crop species can help target below-ground pests and diseases. Typically, this is due to chemicals released post-termination.
Certain brassicas may control nematodes, such as potato cyst nematodes (PCN) or potato root nematodes, which are 1-mm long roundworms.
There are around 12 species that live on the roots of crop plants, such as potatoes and tomatoes. They can cause significant damage, with yield reductions of up to 60% at high population densities.
Carefully selected and managed biofumigant cover crops can contribute to control. This includes consideration of termination times and residue incorporation.
Visit the AHDB web page on this topic
(Image: AHDB)
Weed suppression
The rapid and vigorous growth of some cover crop species can help to suppress weeds.
The release of allelopathic compounds from some plant species – from leaves, flowers, seeds, stems and roots of living and decomposing plant materials – can also influence weed growth and density. Some species produce root exudates that have further allelopathic effects. However, these can potentially impact the following main crop.
Visit the AHDB web page on this topic
Role of Allelopathy in Weed Management: A Review (arccjournals.com) (2020)
(Image: Frontier)
Requirement for stewardship schemes and Sustainable Farming Incentive (Defra)
Ensure that you have the right mixture of species families in the cover crop – a cereal, a legume, and a brassica.
See the Defra website for latest updates and refer to your stewardship agreements.
(Image: AHDB)
How to assess soils (David Purdy, David Purdy Soils)
How to assess soil health on farms
Further insights into assessing soils (David Purdy, David Purdy Soils)
How to assess soil health on farms
The soil health scorecard (Amanda Bennett, AHDB)
How to assess soil health on farms
Examining soil structure (Chris Duller)
The most commonly used online resources for soil health and function have been reviewed by a team at Newcastle University, with the following recommended.
