Carbon key to soil health and productivity

With the Agriculture Bill proposing that farmers will be paid for ‘public goods’, such as improved soil health, growers are being encouraged to start making changes now to help meet these objectives.

Presenting at the QLF Agronomy Soil Event, Joel Williams from Integrated Soils, emphasised the critical role of soil organic carbon (SOC) in improving the health and productivity of UK soils.

“SOC is the cornerstone of soil health – it’s what drives the soil system, including fuelling digestion and feeding soil microbes which release nutrients for plant uptake,” he explained.

“Increasing this vital component can also help to store soil minerals, immobilise heavy metals and pollutants, improve soil structures by building aggregates, protect the soil surface, buffer soil temperatures and improve system resilience,” he said.

“Growers should therefore be working towards increasing SOC by applying, growing and protecting carbon.”

Applying carbon

Mr Williams told delegates that applying carbon is the most common way of increasing SOC, and can be achieved by applying compost, livestock manure, biosolids, anaerobic digestate, mulch or biostimulants, typically prior to drilling.

“Although these inputs contain carbon, they also contain nitrogen, so it’s important to make sure the carbon to nitrogen ratio is maintained,” he explained.

“This is vital because when the carbon to nitrogen ratio isn’t balanced and high nitrogen inputs or amendments are used, microbes then feed on the soil organic matter, respiring carbon dioxide back into the atmosphere, depleting carbon and causing long term soil fertility and environmental problems.

“The best way to achieve this balance, when using organic amendments, is by composting a mixture of brown and green carbon sources, because the brown carbon sources (high in carbon) help to stabilise the green carbon (high in nitrogen).

“Brown carbon sources are stubbles, straw, dry grass, woodchips, newspaper and tree prunings. Whereas green sources are manures, fresh grass, green leaves, vegetable waste, blood and bone, leguminous hay.”

In addition, carbon has another positive soil health benefit when applied.

“Including a carbon source with any applied fertilisers will help to stabilise and buffer the nutrients, particularly when using water soluble inputs. This is because the carbon binds to the nutrients, chelating and complexing them, which improves plant uptake,” he said.

Options include carbon based liquid fertilisers like L-CBF BOOST™, seaweed extracts, plant teas or solid carbon sources such as compost or manures.

This integrated management strategy can help increase nutrient retention, allowing their slow release, reducing leaching and improving nutrient use efficiencies.

Growing carbon

When it comes to growing or photosynthesising carbon, emerging evidence suggests that root exudates are a critical driver behind forming SOC.

“The secretion of root exudates allows the plant to influence the soil biology in its rooting zone,” he said.

“In this process photosynthates, such as glucose and organic acids, are secreted into the soil as root exudates and feed the soil biology that scavenge nutrients for the plant. These root exudates are also captured by the soil microbiota and transformed into stable carbon pools.

“Different plants release different root exudates into the soil and at different depths, so increasing plant diversity to your system can increase the amount of soil biology present to scavenge nutrients, which ultimately leads to less need for synthetic inputs,” said Mr Williams.

“This is why companion cropping can be very effective as multiple plant species are grown at the same time, at different rooting depths and growth stages, meaning there is less competition and more varied root exudates are secreted.”

He also pointed out that companion cropping has the added benefit that ground is not taken out of production to achieve this, as a cash crop and soil improving companion or cover crop can be grown at the same time.

Protecting carbon

“As you’ve worked to increase SOC, through either growing or applying it, it’s therefore vital to ensure that it’s protected,” said Mr Williams.

He explained one of the key ways to protect carbon is to reduce the amount of bare soil between harvest and drilling the next crop.

“When soil is left bare, SOC can be burnt off into the atmosphere. Therefore, having a leafy and diverse cover crop, or ensuring that the ground is covered by a carbon residue, can help protect the soil surface and retain the SOC in the soil.

Read how one farmer has saved £35/ha by planting companion crops with his OSR crop, here.

“The same goes for when you cultivate the soil, anything that disturbs the soil and opens it up to the elements means SOC can be lost to the atmosphere,” he said.

Mr Williams therefore suggested switching to a minimum-till or no-till system to reduce the risk.

“Despite the obvious benefits of no-till, there is still some mixed evidence to its potential for SOC capture and storage,” he said.

“If we measure down to 30cm then no-till soils store more carbon. However, if we measure down to 1m then the difference becomes debatable with no significant difference often shown.

“Therefore, cultivated soils can be as functional as min-till but only if that disturbance is intelligent, meaning using the right tool at the right time, and offset with regular organic amendments.”

He emphasised there is no need to lock heads debating which is better or worse, but rather to work toward improving soil aggregation irrespective of whether tillage or no-till strategies were being implemented.

He also added that improving aggregation can help to hold onto SOC for longer.

“Soil aggregation is influenced by many factors, one of the most important of which is the presence of mycorrhizal fungi (AMF), as found by a long-term study by Wilson et al., in 2009.

“The bi-products from these fungi act as a glue which holds the soil particles together. Cultivation breaks apart these precious aggregates, and the study also found that fungicide applications also reduced the presence of aggregates.”

Mr Williams explained that there are a number of steps that can be taken to enhance mycorrhizal fungi.

“Ensure that soil is always covered and protected by planting cover crops or keeping rotations as tight as possible. It’s also important that the cover crop species are host plants for AMF as this will allow the flow of root exudates which are a food source for the fungi.

“If you’re growing a brassica crop, which aren’t hosts for AMF, consider intercropping with an AMF dependant plant, such as a legume.

“Supporting AMF in the soil via maintaining soil cover, more plant diversity and reducing soluble nutrient inputs by combining them with a carbon source are the primary strategies.

“Secondly it may be worth considering using a direct AMF inoculation in certain situations such as after a non-AMF host in the rotation, or after a bare fallow.

“This could be in the form of a seed treatment, seedling drench or liquid injection, which will ensure AMF are present to scavenge nutrients, hold onto carbon in the soil and increase soil aggregation, all of which can also help speed up crop establishment.”

Joel Williams’ 10 strategies to transition toward agro-ecology

1. Add plant diversity
2. Feed soil biology
3. Manage soil carbon
4. Minimise soil disturbance
5. Remineralisation
6. Reduce synthetic inputs
7. Integrated nutrient management
8. Foliar management
9. Livestock integration
10. Systems thinking

As seen in Agronomist and Arable Farmer.