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

Figure 1.7 The influence of soil type, climate and management factors on potential soilorganic matter content (Ingram and Fernandes 2001).14MANAGING SOIL ORGANIC MATTER: A PRACTICAL GUIDEdecreases in the soil because it becomes tied-up,or immobilised in the microbial biomass. Therefore,residues with a high C:N ratio are considerednutrient poor and can take years to decompose. Incontrast, fresh legume residues with a low C:N ratio(less than 25:1) have proportionally less carbon perunit of nitrogen enabling them to decompose fasterand release surplus nitrogen for plant use.While plant residues and other organic inputsvary widely in their C:N ratios, the C:N ratio of soilorganic matter is generally constant for a givenenvironment (ranging from 10:1 to 15:1). This resultsfrom the dominance of the humus and resistantorganic matter fractions in soil and reflects thesignificant loss of carbon associated with organicmatter decomposition. In Australia, agricultural soilsgenerally store between 10-12 units of carbon forevery unit of nitrogen (i.e. 10 tonnes carbon perhectare for every tonne of nitrogen). Therefore,increasing organic matter (and hence organiccarbon) in soil requires a balance of carbon andnitrogen as well as other nutrients.To maintain or increase a soil’sstock of organic carbon long-term,increased amounts of organicmatter must be continually added.Any decline in the amount oforganic material being returned tosoils will result in a decrease in thesoil’s organic carbon content.When soil organisms digest organic residues partof the carbon originally in these residues is used fornew growth and cell division, with the remainderbeing emitted as carbon dioxide. As a general rule,less than one-third of the applied carbon in freshresidues remains in the soil after the first few monthsof decomposition. As the material is decomposedthe C:N ratio decreases and the remaining organicmaterial becomes more resistant to further decay.