Soil carbon

By: David Walker, Executive Officer
June 11, 2010

Soil organic carbon (SOC) is the basis of the productivity and resilience of natural systems. While some carbon is in inorganic form, such as the limestone nodules in alkaline soil, it is the amount and function of organic carbon that is central to soil health.

The loss of SOC resulting from many of our traditional agricultural practices is the main reason some soils have lost their ability to hold moisture. It is this capacity that enables plants to keep growing longer through dry spells. Rebuilding SOC will make our production systems more resilient to dry spells and a hotter (more evaporative) climate.

So what is it? SOC is the carbon content of soil organic matter (SOM), which itself has come from living organisms, alive or at some stage of decomposition. SOM is generally around 40% carbon, with the remainder being all the other elements that combine with carbon to make complex organic molecules like proteins, amino acids, carbohydrates, etc.

SOC is made up of:

• Partly decomposed organic matter
• Soil microbes
• Humus (which is old organic material that has decomposed to the extent that its source can’t be determined)
• Charcoal (which is burnt OM, in various stages of oxidation)

SOC is classified into ‘pools’ depending on its stability. The least stable carbon is classified as ‘labile’ or ‘fast’, and it tends to move in and out of the soil more rapidly, perhaps less than a year. It is mostly the partly decomposed organic matter and soil microbes. 

The ‘slow’ pool is mostly the humus part of the SOC, which may take of the order of 10 to 100 years to cycle.

The most stable pool of soil C is charcoal, and it is known the ‘recalcitrant’ or ‘passive’ pool. This pool of carbon may stay in the soil more or less unaltered for thousands of years. 

As well as being critical for moisture retention, soil organic carbon is the basis of soil chemical, physical and biological fertility. Chemical, or nutrient, fertility, is the ability of the soil to provide nutrients, in an available form, to plants. This comes from the microbial decomposition of SOM releasing nitrogen, phosphorus and other nutrients for uptake by plant roots. Most of this activity comes from the ‘labile’ and ‘slow’ pools.

The physical fertility of soils relates to the soil structure and the ability of the soil to retain moisture. In the process of decomposition, microbes produce resins and gums that help bind soil particles together into stable aggregates. The improved soil structure holds more plant-available water, and allows water, air and plant roots to move easily through the soil. This activity is also most associated with the ‘labile’ and ‘slow’ pools.

Biological fertility is the ability of the soil to feed its population of microbes, small insects, worms, etc. The availability of SOC controls the number and types of soil biota and their activities which include recycling nutrients, improving soil structure and even suppressing plant diseases. This soil attribute is mostly to do with the ‘labile’ pool. 

SOC can also act as a buffer to adsorb toxins and heavy metals, and help the non-harmful degradation of pesticides. The SOC associated with this is mostly the ‘slow’ and ’recalcitrant’ pools.

The soil carbon that has been lost, tends to be more from the ‘labile’ pool.

SOC then, is very important to productive sustainable farming systems, and anything we can do to build SOC can only be good. The ability of soil to sequester atmospheric CO2 is an added bonus!

More information is available in a very good I&I publication, A farmers guide to increasing soil organic carbon under pastures.

The soil carbon cycle

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