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Soil organic carbon (SOC) is the carbon associated with organic matter in soils, resulting from the decay of organic sources such as plants, animals and microbes. This is distinct from soil inorganic carbon which is held in soil minerals such as calcium carbonates (e.g. limestone)

Soil organic carbon comprises about 58% of the organic matter in soils and is an indicator of soil health. A long term decline in soil organic carbon has a negative impact on fertility, productivity, resilience and mitigation of climate change.

Soil organic matter, has a pivotal role in soil health and a large impact on soil properties. It is important for stabilising soil structure, creating aggregates of soil particles, increasing water infiltration and overall water holding capacity and storage and release of nutrients. It contributes to soil cation exchange capacity and pH buffering capacity. It has a critical role as a food source for soil organisms, increasing their diversity and activity so they can cycle the nutrients and compete with pests and pathogens.

The amount of SOC is a balance between inputs (from plants and micro-organisms) and losses (from natural breakdown and erosion). Rainfall and soil texture are two key factors that determine the amount of carbon that can be grown and stored in soils. The various components of organic carbon have varying degrees of resistance to breakdown.

Measuring soil carbon

Soil tests for soil organic carbon 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 = 1.5g carbon per 100g soil = 15g 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 0.3 x 1.3 x (1.5/100) = 58.5 tonnes carbon per hectare.

Organic matter is difficult to measure so soil organic carbon is used as a proxy. As soil organic carbon comprises about 58% of the organic matter in soils, soil organic carbon is multiplied by 100/58 (or 1.72) to estimate the proportion of organic matter in the soil sample.

For example: 1.2% SOC x 1.72 = 1.5% SOM