Soil carbon

Soil tests for SOC normally report as a percentage, which translates directly as the weight of SOC (in grams) per 100 grams of oven-dried soil (g C/100g soil).

Therefore 1.5% SOC = 1.5 g carbon per 100 g soil = 15 g carbon per kg soil.

Using a measure of bulk density, which is the weight of soil in a known volume, the amount of carbon in tonnes per hectare (t/ha), in a given depth of soil, can be calculated. This is known as the carbon stock which is required for carbon accounting schemes in Australia.

For example, if the soil sample depth is (0–30 cm); bulk density is 1.3 g/cm³ and organic carbon is 1.5%:

SOC (t/ha) = 10,000 x [soil depth in m] x [bulk density] x ([% SOC]/100)

SOC (t/ha) = 10,000 x 0.3 x 1.3 x (1.5/100) = 58.5 tonnes carbon per hectare.

SOM is comprised of various organic compounds (e.g. proteins, waxes, sugars and other complex substance) and nutrients (e.g., carbon, nitrogen, phosphorus, potassium, calcium, and magnesium), depending on its source.

SOM is difficult to measure so SOC is often used as a proxy. Generally, SOM is thought to be comprised of about 50-58% SOC, depending on the type (source) and age of SOM present. SOM content (%) of a soil sample can be approximated by multiplying SOC content (%) with a conversion factor.

For example: % SOM = [% SOC] x [conversion factor]

This conversion factor can vary between 1.4 and 2.5, depending on soil composition.

SOC is also often used as an indicator of soil biological health. But in some instances, such as highly acidic soils where biological activity is generally lower, measurements of soil carbon turnover and soil microbial biomass are better indicators of soil biological health.

Baseline soil carbon stock data obtained from commercial soil testing laboratories and soil mapping datasets were used to estimate soil carbon in the top 30 cm across the South Australian agricultural zone. It was estimated that 320 Mt of carbon stock was present in South Australia’s cleared agricultural lands.

Analyses were also conducted to identify the potential to increase SOC storage in these soils. Regional benchmarks have been established for the state and agricultural districts based on laboratory results from laboratory tests from 1990-2007.

DEW has been in partnership with PIRSA to assess the influence of soil type, rainfall and farming system on the amount and nature of soil organic carbon. A recent study established SOC concentration benchmarks and baselines for the South Australian agricultural zone based on soil test data for the period 1990-2007. The meta-analysis included approximately 36,000 soil test results collated over that period. SOC levels and the proportion of soil samples analysed within low, medium and high SOC ranges for soil texture, land use and agricultural districts were defined.

• As expected, SOC values increased with increasing clay concentration for sand to loam textured soil. However, there was a plateau for clay loam and an unexpected decline in SOC for clay-textured soils. This could indicate potential for improved soil carbon storage if limitations could be overcome and requires further investigation.
• There is opportunity to increase SOC below the topsoil layer under a conducive environment. The subsurface soil (10-30 cm depth) held approximately 75% of the topsoil SOC, while the subsoil (>30 cm depth) held 30%.
• Pasture soils had higher mean and a wider range of SOC concentration by soil texture than cropping soils.
• SOC concentration increased on average by 0.08% per annum (p.a.) during 1990 to 2007. This was largely driven by an increase in SOC of 0.11% p.a. in pasture soils with a smaller but still positive increase of 0.04% p.a in cropping soils.