What options are available to reduce carbon dioxide emitting cultivation?
By farming advisor Jerry Alford
More is now understood about the influence of farming on global warming, particularly methane from livestock and the negative consequences of tillage or cultivation on soil carbon levels.
Cultivations and carbon dioxide
On a worldwide scale, there is no doubt that reducing cultivations can be a factor in cutting carbon dioxide emissions in farming and increasing the sequestration of carbon in the soil. Many farmers are already adopting techniques that reduce the intensity of cultivations and use systems which have a regenerative effect on soil. For an organic arable farmer, a systems approach using diverse rotations, livestock and the incorporation of fertility-building leys has been shown to increase soil organic matter despite using ploughs.
The problem with ploughing
Ploughing is often highlighted as a major cause of soil organic matter loss, particularly when aggressive cultivations are carried out. All cultivations lead to some loss of organic matter and, although we all try to select the best cultivators for the soil type and conditions, there is no one size fits all, nor the perfect machine. Potentially bigger problems are created by deep cultivations and destoners in vegetable systems, particularly with rotations which are based around continuous winter-sown crops, with no attempts to increase or maintain organic matter.
Adding organic matter
One technique to mitigate the effects of cultivations is to add more organic matter. In mixed farming systems this often comes from livestock manures and the grass leys in rotation. However, composts, green manures and chopping straw instead of baling can all help maintain healthy soils.
The benefits from green manures come not only by adding organic matter to the soil, but also by keeping healthy biological activity in the soil. This is achieved by the movement of carbohydrates from photosynthesis into the soil to be used by bacteria and fungi, in return for nutrients, such as phosphorus and potassium.
Min-till and no-till
Non-inversion methods, such as min-till, are also being looked at by organic farmers, but there are a range of machines and techniques in operation. Many non plough based systems are referred to min-till, but generally min-till is defined as shallow, under 100 mm depth.
Where soil type allows, shallow min-till can be applied in organic systems as a method to prepare seedbeds, particularly those with low weed populations. Shallow min-till is a good way to minimise loss of organic matter from the soil, but it may not be as effective at weed control.
No-till methods don’t involve cultivations. When combined with the use of cover crops, no-till is known as conservation agriculture.
Cover crops and green manures
The use of cover crops or green manures is a common practice in conservation agriculture systems and now many farmers are looking to incorporate livestock into their arable systems. There is an Innovative Farmers field lab looking at the benefits of livestock to soil health.
Consequences for organic systems
For some organic farmers, ploughing in green manures has been part of their systems fertility, recycling nutrients and improving soil health as part of a stockless organic systems. Many conservation agriculture farmers are basing their strategy around cover crops controlled by herbicides, whilst maintaining a relatively high input system, with the only soil movement being directly around the seed at drilling. For organic farmers there are obvious difficulties with this type of approach, particularly the use of herbicides and fertilisers, but there are benefits to adopting some of these alternative strategies.
Innovative Farmers field lab looks for solutions
One secondary consequence of cover crops is their role in competing with those weeds which germinate after harvest. As part of a weed-control strategy, crops such as buckwheat have been shown to be useful against perennial plants like couch grass (Elymus repens), shading the grass while it is trying to regrow after cutting and is at its least competitive. Results of an Innovative Farmers field lab showed buckwheat was not totally effective in controlling couch grass, although it did reduce it. Of more significance was the effect of buckwheat on the soil compared with the alternative treatment of continuous cultivations known as a bastard fallow. Buckwheat left the soil with better structure, aggregation of soil particles and water infiltration. This created an environment that was less suited to couch, which prefers compacted and poorly structured soils, and more favourable to the next crop.
A further benefit associated with cover crops is their role in providing roots which have arbuscular mycorrhizae. These fungal symbionts are associated with nutrient sharing in the soil and they remain active through winter to spring when the next crop is planted. Although brassicas do not normally develop strong associations with mycorrhizae, mixed cover crops keep this important element of the soil food web going.
Research shows that arbuscular mycorrhizae are not as affected by cultivations as previously thought, particularly when associated with overwinter cover crops (Journal of Applied Ecology, 2016, 54, 1785– 1793). This may be linked with the addition of organic matter to the soil profile, which the fungi need as a nutrient source.
The potential for organic no-till systems is dependent on finding methods which achieve effective weed control, minimal soil movement and return of nutrients. One often mentioned technique involves crimper rollers, which crush the stems of the cover crops.
This is effective when crimping coincides with anthesis (flowering). In addition, crimpers are being incorporated into drills which work along with herbicides in a no-till situation. The Innovative Farmers trial, Alternatives to glyphosate for terminating cover crops, showed that a standard Cambridge roller was effective when there was a frost. Unfortunately, the timing of drilling when there is a frost, or when a cover crop, such as rye, is at anthesis means that this is not yet a proven option for organic farmers and more research on suitable cover crop mixtures is needed. The mulch left on the soil surface has been shown to be effective at preventing weed growth.
Use of living mulches
A new Innovative Farmers field lab, No-till with living mulches, is getting underway. It is looking at the potential of using a permanent understory of white clover as a cover crop and living mulch. The premise is similar to the pasture cropping system which has been used in Australia.
The permanent understory is managed through the season to provide weed suppression and nutrients, and potentially a food source for livestock, whilst producing a cereal cash crop. Clovers are already used as understorys in beans and the practice of undersowing cereals is a common organic technique. The cash crops will be sown into either a grazed or crimped clover crop in the autumn at a time when the clover’s growth is slowing. In spring, the large biomass of the crop should out compete the clover and keep growing. Nitrogen will be released by managing the clover, possibly by mowing or crimping between rows. The clover mulches are currently being drilled and the full trial will start in the autumn.
The build-up of weeds is the most likely factor hindering the full uptake of no-till but in principle, the idea of reduced ploughing in organic crops by using all the available tools does allow organic farmers to further increase soil organic matter and carbon sequestration.
First published in Organic Farming Magazine, Spring 2020
Find out more
Innovative Farmer field labs:
- Alternatives to glyphosate for terminating cover crops
This field lab is investigating alternatives to ploughing or glyphosate for terminating cover crops. The group are trialing a range of techniques including roller crimpers.
- Buckwheat for couch control
This field lab is investigating the potential for buckwheat (and other crops) to reduce couch grass infestations.
- No-till with living mulches
This field lab investigates the potential for establishing no-till organic/low input arable farming systems using a permanent living mulch understory.
- How to manage living mulch growth
This article looks at the benefits of permanent living mulches on soil cover year-round. No-till and reduced till drilling are also possible directly into the mulch saving time, fuel, and labour.