Last month I attended the Healthy Soils Symposium on the Sunshine Coast along with farmers, soil scientists and many others who want to know more about soils.

There was a lot of interest in soil biology and soil carbon, particularly the opportunities for farmers to trade in carbon by storing more carbon in their soils.

Carbon is the new black, as the saying goes, because industries that produce carbon emissions are looking at agriculture and forestry to store carbon in soils and vegetation and hence provide ‘carbon credits’.

However, it became apparent to me the global interest in carbon sequestration and carbon credits has outdistanced many people’s understanding of where and how carbon fits into the system.

As one farmer said to me at the symposium, ‘I understand NPK, it has been drilled into us, but where does carbon fit in - where does it come from, what does it do?’.

It’s beyond the scope of this column to outline everything farmers want to know about carbon, but here’s a ‘rough guide’.

Carbon is the sixth most abundant element in the universe after hydrogen, helium, oxygen, neon and nitrogen.

Its atomic structure means it can form strong bonds with many other elements. In fact, carbon atoms have an almost infinite capacity to bond in long chains or rings.

When carbon atoms bond with each other in three dimensions they form diamonds. When they bond in two dimensions they form graphite. When they bond with hydrogen, oxygen or nitrogen atoms they form organic compounds such as proteins and carbohydrates.

All plant and animal cells are based on compounds of carbon and other elements.

Geological sources of carbon include coal, oil, natural gas, and limestone, all formed from ancient plant materials.

In their gaseous state carbon atoms bond with oxygen to form carbon monoxide and carbon dioxide, and with hydrogen to form methane.

Carbon dioxide levels in the atmosphere have increased because burning coal and oils releases carbon into the atmosphere.

In soils carbon is found in living organisms and decomposed plant and animal material, and is available in different pools in the soil.

The active or labile pool contains organic matter that is currently decomposing into plant available nutrients. It includes crop residues, dead plants and animals, mulch, compost etc. This carbon is the main food source for microorganisms and most of it is breathed out by the organisms as carbon dioxide.

Labile carbon is only in the soil for a short time, from a few months to a few years, so needs to be continually replaced.

The intermediate or slow carbon pool is mainly humus, a product of microbial decomposition and a complex carbon structure that does not decompose easily and can last in the soil for decades if not ploughed up and exposed to air and microbial attack.

The recalcitrant pool is carbon that is very stable and does not decompose for hundreds or thousands of years. This pool includes charcoal and charred material; the charring process forms long carbon chains that are very resistant to decomposition.

Carbon is very important for agricultural soils because it holds water, improves cation exchange capacity and provides food for soil organisms whose activities improve soil structure and make nutrients available to plants. I

t is not easy to build carbon in the soil. It requires a lot of organic material added regularly. For this reason there is now a lot of interest in making ‘agrichar’ from organic wastes to build soil carbon levels in nutrient-depleted soils.

If you’d like to know more about the soil carbon pools there is an excellent summary by CSIRO soil scientist Jeff Baldock in the symposium proceedings at HSSF Symposium (2.9 mb, PDF).