Soil Organic Carbon

Soil Organic Carbon

Carbon in agricultural soils

Storage of carbon in agricultural soils through appropriate management actions has been recognised as an important tool to mitigate climate change. Carbon is the main element present in soil organic matter, on average making up 58% by weight.

Increased carbon storage in soil can:

  • improve long-term farm productivity
  • improve soil health
  • increase the land’s resilience to a more variable climate
  • help enhance Australia’s natural environment
  • protecting against soil erosion
  • act as a water purifying medium
  • act as a sink and ameliorating agent for organic wastes and environmental pollutants
  • support biogeochemical-nutrient cycles.

It is estimated that worldwide, soils contain approximately double the carbon present in the atmosphere and approximately three times the carbon contained in living plants and animals. It has also been estimated that globally, agricultural ecosystems have lost 50% to 70% of their natural soil organic carbon pool. The greatest accumulation of soil organic carbon typically occurs in the surface soil, with this organic enriched layer often 10 cm or less thick in South Australian soils. Therefore, it is important to identify and implement practices that minimise or reverse this decline in soil organic carbon, whilst balancing economic sustainability and global food needs.

Soil organic carbon

The amount of organic carbon in soil is a balance between inputs (plant and animal residues) and outputs (decomposition/mineralisation, erosion and removal of plant and animal products). Soil carbon can be built up with good management practices and a favourable climate, but it can also be lost as a result of some management practices and during times of drought.

Soil organic carbon is the carbon found in organic matter and includes all living and dead organic material in the soil, such as:

  • plants
  • soil organisms
  • animal materials.

Soil organic carbon does not include fresh or undecomposed plant material greater in size than 2 mm or inorganic ‘carbonate’ matter. The inorganic carbon fraction of the soil is derived of weathered rock particles.

There are three pools or fractions of soil organic carbon:

  • particulate - primarily comprises fresh plant and animal materials that are easily decomposed
  • humus - materials that are less than 0.05 mm in diameter, and can persist in the soil for many decades, and store and release nutrients that can be directly utilised by plants and microbes
  • resistant/recalcitrant/inert - charcoal and charred remnants of organic materials.

An adequate amount of soil organic carbon is important for a healthy community of soil microbes that provide many benefits to plant growth. Microbial activity, supported by soil organic carbon, helps turnover of nutrients and deliver them in plant-available forms. Microbial activity can provide disease-suppressing capacity to soils, through competitive exclusion and diverse defensive mechanisms against potential pathogens.

" Your soil type determines the potential storage of organic carbon. The climate determines the attainable storage of soil organic carbon. Your land management determines the actual storage of organic carbon in soil."

‐ Jennifer Carson, The University of Western Australia

How you can increase soil organic carbon in the Limestone Coast

The potential for storing more carbon depends on a soil’s measured base level of organic carbon and its storage capacity. Climate, soil type and condition, as well as land use and management practices affect the amount of organic matter that can be grown and put into the soil.

Ultimately, the opportunity to increase soil organic carbon depends on the:

  • ability to grow or supply sufficient biomass (mostly in the form of plant roots) and maintain these inputs, otherwise soil carbon can also be lost
  • ability of the soil to stabilise organic carbon (in combination with clay particles and soil aggregates)
  • capacity of the soil to store more organic carbon
  • capacity to convert organic carbon to more stable fractions.

Existing soil organic carbon stocks

Soil Organic Carbon
Soil carbon stocks for the Limestone Coast top 30 cm.