Trees and shrubs are very effective at capturing carbon and are widely recognised as an important tool in climate change mitigation. But grassland also has a major role to play and the combination of sustainable farming and meeting net zero targets depends largely on what’s going on below the sward – in the soil.
Sarah Bolt, membership development manager at Kingshay Dairy Specialists, recently compiled the Soil Carbon Report, combining the key findings from Kingshay’s own Soil Organic Carbon Project with other research and knowledge.“ Grassland has huge potential for carbon sequestration and, unlike woodland which stores the majority of sequestered carbon in its vegetation, it stores up to 90% of captured carbon within the soil as soil organic carbon (SOC),” she says.
Carbon in our soils goes beyond climate change mitigation; it is the foundation of healthy, fertile soils.
And carbon in our soils goes beyond climate change mitigation; it is the foundation of healthy, fertile soils – providing good structure, chemistry, and biology. The healthier and more stable our soils, the more productive and resilient to challenges like leaching, drought and erosion caused by the changing and extreme weather patterns increasingly seen in the UK.
“The soil carbon project investigated current SOC levels in UK grassland and the main influences over carbon sequestration and retention,” explains Ms Bolt. “We then published the report as a practical guide for farmers who want to better understand SOC and how to measure and manage levels in their grassland soils; all while helping farmers achieve healthy soils and productivity well into the future.”
So what do farmers need to consider?
Soil Organic Carbon (SOC) and Soil Organic Matter (SOM) are two terms that frequently crop up in conversations about soil health and carbon sequestration, says Ms Bolt. SOC is a component of SOM: The latter predominantly comprising carbon alongside water, nutrients like nitrogen and phosphate, and soil organisms and their secretions.
“For a long time we’ve measured SOM to determine carbon levels, believing SOC to be around 58% of total SOM,” she explains. “But we now have a better understanding of what determines and influences the amount of carbon stored in soils, and that it can vary greatly from farm to farm.”
So while farmers can translate ‘good’ to ‘very good’ SOM levels as good SOC – all the different components of SOM means that any estimation of carbon level is at best a guess.
One of the greatest factors affecting carbon levels is soil type. “There is a positive correlation between the soil clay content and SOC,” says Ms Bolt. “Clay particles are very small and can pack themselves together to form large surface areas which hold SOC in electrostatic bonds – forming aggregates.
“This stabilises the organic matter which contains the carbon, keeping it from being lost as carbon dioxide (CO₂) through decomposition by soil organisms.” In contrast, it can be hard to reach high SOC level in sandy soils. “Sandy soils are a challenge because they do not have such a large surface area as the clay particles, which means SOC is not stable and isn’t held well within the soil,” she explains.
“It doesn’t matter how much organic matter you add to a sandy soil – it has a limited SOC capacity and once capacity is reached farmers need to change their mindset from ‘building levels’ to ‘maintaining levels’.”
The percentage of SOC to the percentage of clay is referred to as the SOC/clay ratio, and all managed soil types should be able to achieve a minimum ratio of 0.1. And while figures can benchmark an individual farm’s SOC, it doesn’t clearly show how well the soil – for its type – is performing. To tackle this issue, a group of researchers developed an index system for SOC/clay ratio which separates soils into four categories: Degraded (<0.077), moderate (<0.1 ≥0.077), good (<0.125 ≥0.1), and very good (≥0.125).
“A sandy soil is always going to hold less SOC than a clay soil so we use the index categories to clearly show the relevance of that SOC/Clay ratio to the soil texture,” she explains.
Results from the 100 farms that took part in the project showed an average SOC/clay ratio of 0.15, but ranged from 0.06 to 0.54. In terms of categories, 59% of soils under permanent pasture were very good and 27.9% were good, while soils under short to medium term grass leys scored 36% and 41%, respectively. “It is worth noting that more soils are categorised as degraded or moderate under temporary grass leys (23%) then under permanent pasture (13.1%).”
It is widely accepted that to manage you need to measure – and the same goes for carbon levels in soils. “Measuring will give you the current status and starting point,” says Ms Bolt. “From that you can begin to benchmark, plan, and implement appropriate action.”
Measuring SOC levels can be done two ways: Loss of Ignition (LOI) testing – a traditional method used to measure SOM that assumes SOC accounts for 58% – and the Dumas Combustion method – a specific and most accurate measurement method. “It’s important for farmers to be clear about the differences, because while LOI testing is cheaper it does not give accurate results. Field sampling followed by lab analysis using the Dumas Combustion is considered the gold standard.”
Of course, it depends what farmers need to know; is it just for interest or improving soil health? To help mitigate climate change by increasing carbon levels or with a focus on GHG offset and carbon trading? It is worth repeat testing every three to five years alongside soil bulk density analysis to assess how changes in soil conditions are affecting SOC levels. This will also help farmers who want to better understand carbon stocks in their soils – with a view to potential offsetting and trading – by establishing a figure for, and trend of, tonnes of carbon per hectare (t/ha).
The golden rules for soil sampling are: Test in autumn, avoid testing fields ploughed in the previous six months, avoid testing after fertiliser or manure applications, and be consistent in the laboratory and method used.
Once farmers have the right information about their soils and index category they can tailor their actions accordingly. “Degraded or moderate soils need SOC levels built while very good and good soils need their levels protected,” stresses Ms Bolt.
To this end, the mindset needs to change around ploughing. “We need to think about how and why we reseed – ploughing is still the main method of cultivation,” she explains. Reducing ploughing and incorporating new cultivation techniques maximises soil health and increases productivity.
Best practice pasture management like optimising pH, using high dry matter organic manure like farmyard manure, and targeting chemical applications will help replenish SOM and SOC – especially on grasslands used for silage production.
And grazing has a role to play. “Grazing livestock can be both beneficial and damaging; overstocking and poaching will lead to soil degradation,” says Ms Bolt. “Rotational grazing is good for incorporating organic material – but it also improves grass productivity and longevity, meaning there is less need to reseed and disturb soils.”
Also worth serious consideration is sward diversity. “Increase diversity by introducing multi-species swards, particularly legumes like clover and deep-rooted species like chickory or plantain. “It is estimated that soils take 20 to 50 years to reach an equilibrium from being ploughed to reaching maximum soil carbon levels,” stresses Ms Bolt. “So there isn’t time to delay – farms will productively and economically benefit much more from taking action sooner rather than later.”
Case Study: Locking CO2 in the soil with dairy farming?
For more than 30 years, the Collingborn family has been running their farm in Chippenham, UK, monitoring the carbon content in the soil. In a case study, they report on their approach and the experience they have gained over many years.