Q88. How do I add more carbon to my soil?

Without physically trucking organic matter in, such as manure from another farm, increasing soil carbon (and soil organic matter) largely comes down to paddock management. At a macro level, it’s a simple concept—to increase soil carbon, the rate of carbon addition needs to exceed the rate at which carbon is lost through erosion and microbial decomposition. The realities of doing so are a whole other matter, but here is a short version of what we know can work.

1. Maximise crop productivity and root growth – More plant growth, especially root growth, puts more photosynthetic carbon into the soil. Roots increase deeper soil organic carbon (SOC) through their own biomass, releasing exudates, and sloughed-off cells. The organic carbon (OC) in the area surrounding the root (rhizosphere) is much higher compared to the bulk of the soil, and roots contribute significantly more OC to the soil – 2 to 6 times more – than aboveground shoot residues. Root exudates also promote microbial activity deeper in the soil, helping transform OC into more stable forms.

Strategies to maximise crop productivity and root growth will vary for every situation, but common strategies include:

• Removing constraints to root growth (point 5 below)
• Use rotations with different species such as legumes, pastures, and cover crops to increase the variety and depth of root carbon inputs. Soil organic carbon closer to the surface, in the top 10 cm, is more likely to be decomposed and cycled by microbes, while deeper OC is more likely to be stored.
• Consider using varieties with a range of root architectures.
• Optimising nutrient availability to support microbial growth which is essential for OC cycling and transforming labile carbon into stable forms. One tonne of humus-carbon requires, among other things, 80kg of N, 20kg P and 14 kg S above crop needs.

2. Limit erosion – As a large proportion of organic matter is present in the top 0-10 cm of soil, protecting the soil surface from erosion is essential for retaining soil organic matter.

3. Use pastures – One of the most effective ways to increase soil carbon is with pastures. Pasture roots grow deeper and often for longer than crops contributing more organic matter. Pastures are also typically disturbed less than annual crops, while grazing adds manure and stimulates plant growth and associated carbon inputs.

4. Stubble retention – Retaining stubbles keeps them as a source of organic matter to eventually become soil organic matter. It also creates a more favourable environment, holding on to moisture longer and reducing soil temperature compared to bare ground. This is good for microbial activity and soil health. It also limits erosion and promotes water infiltration, which helps crop growth and its carbon inputs.

5. Manage soil constraints – Soils with constraints such as acidity and non-wetting have less crop growth and microbial activity. This slows organic matter cycling, means less crop carbon inputs, and slows the rate at which organic matter is converted into more stable forms.

6. Limit soil disturbance – Tillage causes an immediate loss of soil organic matter by exposing previously protected organic matter to microbial decomposition. That being said, if soil constraints are limiting crop productivity (point 1), the benefit of strategic tillage to ameliorate the constraints and improve crop growth will outweigh the minor impact of irregular tillage.

7. Manage grazing intensity – Overgrazing reduces groundcover and crop inputs of soil carbon and increases erosion risk. Well-managed grazing can encourage root growth and stimulate plants to release root exudates.

8. Cover crops – Cover crops protect the soil from erosion, are a further source of crop carbon inputs and feed the microbes. Question 64. ‘Will summer cover crops take all my soil moisture?’ explores when summer cover crops can work well.

See The Effects of Management on Soil Function fact sheet and the booklet 10 Ways to Build Soil Carbon for more detail on ways to optimise OM inputs and minimise OM losses.

Note there is an upper limit on practical carbon storage, which is covered in Question 89. ‘How much carbon can I add?’