The importance of carbon in the soil

The importance of carbon in the soil

The carbon cycle is a fundamental part of life on earth. ‘Soil organic carbon’ (SOC) – the amount of carbon stored in the soil is a component of soil organic matter – plant and animal materials in the soil that are in various stages of decay.

Soil organic carbon is the basis of soil fertility. It releases nutrients for plant growth, promotes the structure, biological and physical health of soil, and is a buffer against harmful substances.

Soil organic carbon is part of the natural carbon cycle, and the world’s soils holds around twice the amount of carbon that is found in the atmosphere and in vegetation. Organic material is manufactured by plants using carbon dioxide from the air and water. Plants (and animals, as part of the food chain), die and return to the soil where they are decomposed and recycled. Minerals are released into the soil and carbon dioxide is released into the atmosphere.

Soil organic carbon accounts for less than 5% on average of the mass of upper soil layers, and diminishes with depth. According to the CSIRO, in rain-forests or good soils, soil organic carbon can be greater than 10%, while in poorer or heavily exploited soils, levels are likely to be less than 1%.1

The amount of soil organic carbon present in soil can vary hugely according to soil and landscape types,an and c climate an change in the same paddock over time depending on climate and farming methods. Temperature, rainfall, land management, soil nutrition and soil type all influence soil organic carbon levels.

In Australia, soil carbon levels have dropped by up to half of pre-agricultural levels in many areas because of activities such as fallowing, cultivation, stubble burning or removal and overgrazing.2

Increasing soil organic carbon has two benefits – as well as helping to mitigate climate change, it improves soil health and fertility. Many management practices that increase soil organic carbon also improve crop and pasture yields.

According to the NSW Department of Primary Industry, although there is a limit on the amount of organic carbon that can be stored in soils, the large losses in the past means that many of Australia’s agricultural soils have the potential for a large increase in soil organic carbon.3

If more carbon is stored in the soil as organic carbon, it will reduce the amount present in the atmosphere, and help reduce global warming.4 The process of storing carbon in soil is called ‘soil carbon sequestration’.

Some of the practices that increase soil organic carbon include conservation farming (reducing or eliminating tillage and retaining stubble from previous crops), improving crop management (e.g. through better rotation), maintaining and improving tree/forestry management, improving grazing management and adding organic materials such as composts and manures. For more details see win-win carbon farming practices.

For further information see NSW Department of Primary Industry’s Increasing Soil Organic Carbon fact sheet and CSIRO’s information about soil organic carbon.


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