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

Table 1.2 Functional role of soil organic matter.Physical functions Chemical functions Biological functionsImproves soil structural stabilityIncreases capacity tohold nutrients (i.e. cationexchange capacity)Influences water retention Buffers pH Major store of plant nutrients(N, P, S)Energy (food source) for biological processes such as:Microbial decomposition; Nutrient transformation; Degradation of pollutantsBinding soil particles and organic matter in stable aggregatesBuffers changes in temperatureImmobilises heavy metalsand pesticidesImproves soil resiliencehumus can be seen to be equally important forthe provision of nutrients across all soil types andin particular it is critical to the supply of potentiallymineralisable nitrogen (see Figure 1.5).is not known how this relationship would differ whenconsidering clay applied to soils for the treatment ofnon-wetting. 12MANAGING SOIL ORGANIC MATTER: A PRACTICAL GUIDEFigure 1.5 A conceptual representation of therole of soluble, particulate, humus and resistant(inert) organic matter fractions for a range of soilfunctions (from Hoyle et al. 2011).While not all of the relationships depicted inFigure 1.5 have been quantified for Australiansoils, the contribution of the soil organic carbonfraction to the ability of a soil to hold nutrientshas been demonstrated at Young River, WesternAustralia (see Figure 1.6). This study suggests astronger relationship between soil organic carbonand nutrient exchange in soils, with less than 10per cent clay and organic carbon explaining nearly40 per cent of the variation in cation exchange. Insoils with greater than 10 per cent clay, a one percent increase in organic carbon increased nutrientexchange by about 3 meq/100 g soil, but explainedjust six per cent of the variation in higher clay soils. ItFigure 1.6 The influence of soil organic carbon oncation exchange capacity for Young River, WesternAustralia in soils with clay content between 0-10%(o), 10-20% (x) and 20-30% ( ). Sourced fromwww.soilquality.org.au.While the relative importance of any given fractionof organic matter will vary from one soil to anotherand depend on factors such as climate andcropping history, we do know that organic matterinfluences plant growth primarily through its effecton the physical, chemical and biological propertiesof the soil.For example, fresh crop residues which are readilybroken down provide energy for key soil biologicalprocesses such as nutrient cycling. The particulateorganic matter fraction decomposes at a slowerrate than crop residues and is important for soilstructure, energy for biological processes andprovision of nutrients. Humus generally dominates