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.
Soil function
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.
(Image: Yorkshire Agricultural Society)
Improving infiltration
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.
Reducing 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).
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.
(Image: AHDB)
Livestock grazing
Cover crops can be grown for short-term grazing fodder for sheep or cattle.
For grazing, consideration must be given to selecting the relevant species that are suitable for animal feed.
Grasses and cereals (such as oats, rye and rye-grass) can all be grazed and deliver good early ground cover (which is important where erosion is a concern), as well provide other benefits, including vigorous rooting.
Autumn-sown brassicas (such as mustards, radishes, and turnips) and legumes (such as sainfoin, vetch and clovers), which fix nitrogen, can also be grazed, with care.
The former often provide good ground cover and deep rooting, with the latter benefitting following crops through raised fertility.
Chicory is a deep-rooted herb (delivering soil structure benefits) and is better suited to longer-term use, especially where grazing is of interest.
(Image: AHDB)
In some cases, grazing cover crops has been shown to improve the cycling of their nutrients, making some nutrients more available to the following crop and leading to an improvement in crop yield.
There are some agronomic issues that could occur from grazing over winter cover crops, such as creating issues with soil compaction and leaching of nitrogen deposited in urine (especially in adverse weather conditions). Such risks may be mitigated by having a ‘back-up’ field nearby, ready to move the livestock into if ground conditions worsen.
If livestock are not already present on the farm, then consider working with graziers or neighbouring farmers who could supply the stock, infrastructure and expertise. Forming a good relationship is key to ensure both parties benefit. This way you can also communicate easily and honestly about the removal date, bad weather strategies, the residual crop target and the species mixture.
See the selection section of the website for information on grazing-friendly species.
Resources
The most commonly used online resources for grazing and animal feed have been reviewed by a team at Newcastle University, with the following recommended.
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.
(Image: AHDB)
The Nitrogen release from Cover Crops (NiCCs) project* has investigated the legacy effect for subsequent crops in the rotation at two trial sites.
Where a cover crop mixture was in place, there was a significant reduction in nitrate leaching (compared to over-winter stubble). Additionally, spring soil nitrogen supply (SNS) was up to 30kg/ha greater.
Further work is required to establish the fate of this nitrogen following the destruction of the cover crop, and whether this translated into a reduction in fertiliser requirements for following crops.
*A collaborative project with ADAS, Affinity Water and Portsmouth Water.
Biodiversity
Above ground
Many cover crop species provide an autumn/winter flowering resource for numerous insects that feed on either nectar or pollen.
Other insects and animals, such as birds and mammals, also benefit from the non-floral food, habitat, shelter and protection that cover crops provide (particularly during the winter months).
As more diverse vegetation can increase the abundance of natural enemies of pests, cover crops may also improve crop yields.
Care must be taken 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
Many of the organisms that make up the soil food web carry out vital below-ground functions, such as decomposition of soil organic matter (SOM) and transformation of nutrients into plant available forms.
Cover crops also provide living plant roots and sugary exudates that serve as habitat and food for beneficial organisms, such as mycorrhizal fungi. They can also support wider soil microbial communities and increase overall microbial biomass.
Soil macro fauna, particularly earthworms, mix soils and form channels that assist with water infiltration and crop rooting. Cover crops may help increase populations of these species, especially earthworms. However, the effects depend on other factors, such as the tillage approach.
A good number of earthworms* are beneficial for soil function. You want a balanced community structure of earthworm species and ages, otherwise some earthworm-mediated soil functions could be compromised.
* More than 8 earthworms per soil pit represent an ‘active’ population for arable soils.
(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.
(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.
Role of Allelopathy in Weed Management: A Review (arccjournals.com) (2020)
(Image: Kings Crops/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)
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David Purdy, David Purdy Soils, further insights into assessing soils
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Amanda Bennett, AHDB Senior Environment Scientist, introduces the soil health scorecard.
How to asses soil health on farms
Chris Duller looks at soil structure
Resources
The most commonly used online resources for soil health and function have been reviewed by a team at Newcastle University, with the following recommended.