managing soil organic matter - Grains Research & Development ...

11MANAGING SOIL ORGANIC MATTER: A PRACTICAL GUIDESoil organic carbon is a component of soilorganic matter, with about 58 per cent of themass of organic matter existing as carbon.Therefore, if we determine the amount ofsoil organic carbon in a sample and multiplyit by 100/58 (or 1.72) we can estimate theproportion of organic matter in the soil sample:Organic matter (%) = total organiccarbon (%) × 1.72While the ratio of soil organic matter to soil organiccarbon can vary with the type of organic matter, soiltype and depth, using a conversion factor of 1.72generally provides a reasonable estimate of soilorganic matter suitable for most purposes.Calculating soil organic mattercontent in soilThe amount of soil organic matter in your soilcan be calculated as follows for a soil with 1.2per cent soil organic carbon:(Total organic carbon per cent x 1.72) x soilmass (tonnes per hectare)= [(1.2 x 1.72)/100] x 1200 (for a soil with abulk density of 1.2 to 10 cm depth)= 24.8 tonnes organic matter per hectareSOIL ORGANIC MATTER FUNCTIONOrganic matter renders soils more resilientto environmental change and also influencescharacteristics such as colour and workability.It is central to the functioning of many physical,chemical and biological processes in the soil. Theseinclude nutrient turnover and exchange capacity,soil structural stability and aeration, moistureretention and availability, degradation of pollutants,greenhouse gas emissions and soil buffering (seeTable 1.2).An optimal level of soil organic matter is difficultto quantify because the quality and quantity ofdifferent organic matter fractions needed to supportvarious functions varies with soil type, climate andmanagement. However, it is generally consideredthat soils with an organic carbon content of lessthan one per cent are functionally impaired.Soil function is influenced by the size, qualityand relative stability of the four soil organic matterfractions (see Figure 1.5). In this figure, the widthof the patterned or shaded areas within the shapeindicates the relative importance of the soil organicmatter fractions to a particular function or process.For example, on the left the decreasing width ofthe striped area indicates the importance of thehumus fraction declines as the clay content of asoil increases. This is because the clay particlesprovide a large surface area for cation exchange,which renders soil organic matter increasingly lessimportant as clay content increases. By comparison,