Soil carbon is a hot topic at the moment, but what do we know about it.  Soil carbon or organic matter has many roles within the soil, and can change the chemical, physical and biological properties of our soils. Soil carbon, due to it properties, contributes to the long term resilience of our soils and farming systems.
 
Soil carbon can change chemical reactions in the soil by acting as a buffer in our systems, changing the availability and exchange of nutrients. Physically soil carbon can change water retention (water holding ability), soil structure and soil temperature. Biologically soil carbon provides energy for the soil microbial population and changes the availability of nutrients like nitrogen, phosphorus and sulphur (nutrient cycling).  There are strong interactions between the chemical, physical and biological interactions of soil carbon, which we do not fully understand.

Disease suppressive soils have a greater microbial activity so are able to compete with disease pathogens in the rhizosphere (the area directly around the plant roots) and decrease the level of infection and disease. Healthy microbial populations are encouraged by stubble retention as carbon is the food source.

Soil organic carbon is comprised of different materials which have different properties and breakdown in the soil environment at different rates. 

The biological types of soil carbon are;

* Crop residues - material greater than 2mm,
* Particulate matter - plant material smaller than 2mm,
* Humus – organic matter of different molecules (also called the ‘glue’ of soils)
* Recalcitrant organic matter – biologically stable and often ‘old’ organic matter like charcoal.

Each of the fractions of soil organic carbon contributes to the soil properties (Baldock, 2007).

Previously soil organic carbon has been given a total value, but new research by Jeff Baldock, CSIRO, has shown the different types of organic carbon vary significantly in agricultural soils and can change with management practices.  In cropping systems humus was the greatest component, compared to particulate matter being the greatest component in pasture systems.  Charcoal can account for between 0-60% of the organic carbon present in the soil, but this fraction is generally relatively stable (Baldock, 2007).
 
Future research as part of the new Eyre Peninsula Farming System project will help us understand and learn more about the types of carbon present in our farming systems on Eyre Peninsula and the importance of organic carbon within our farming systems.

REFERENCE: Defining Soil Carbon, Jeff Baldock et al, The Healthy Soils Symposium, 2007, pg 67-78.

AUTHOR:  Amanda Cook, Eyre Peninsula Farming Systems Project Coordinator - Minnipa Agricultural Centre, SARDI. 

CONTACT:  Amanda Cook, Minnipa Ag Centre, telephone 08 8680 5104.