Ground Cover Supplement : GC Supplement - Nutrient management
3 NUTRIENT MANAGEMENT GROUND COVER Fractions take a piece of the action SOIL ORGANIC MATTER IS A MIXTURE OF MATERIALS THAT DIFFER IN THEIR RATE OF DECOMPOSITION AND FUNCTION. THIS COULD BE PIVOTAL TO PREDICTIONS OF FERTILISER REQUIREMENT FIGURE 1 EFFECT OF SOIL ORGANIC MATTER COMPONENTS ON DETERMINING SOIL CHARACTERISTICS IN RELATION TO CLAY CONTENT 0 20 40 60 80 100 Clay content (%) Clays Sands Soluble Particulate Humus Resistant CEC Soil structure Energy for biological processes Provision of nutrients Soil thermal properties NUTRIENT SUPPLY AND DEMAND production that can be supported by rainfall. Soil type and management were identified as factors that influenced the proportion of carbon in each fraction. “We have established that the amount of each organic matter fraction varies significantly across soil types and some fractions can be altered by management practices,” Dr Baldock says. “This is important as different fractions decompose at different rates and contain different quantities of nutrients.” Due to their rapid rates of decomposition, crop residues generally accounted for the smallest proportion of organic carbon, irrespective of soil type or production system. However, no-till systems resulted in more surface plant residues but had little effect on the other fractions. Under pasture systems, the regular inputs of organic matter from decaying top and root material, combined Jeff Baldock (left) and Kris Broos use analytical equipment to measure carbon dioxide released from different fractions of organic matter. This information has helped them to link organic matter fractions with different soil functions. THe Single Soil organic carbon percentage reported on soil tests disguises the fact that soil organic matter is not uniform but is composed of a wide range of materials with different chemical and physical properties. Research by Dr Jeff Baldock and Dr Kris Broos and colleagues at CSiRo is looking at how soil organic matter varies across soil types, and how these differences interact with nutrient cycling and production management. This article reports their findings in relation to soil nutrition. Soil organic matter, generally measured as soil carbon percentage, contributes to a variety of soil functions, broadly classified into physical, chemical and biological. Strong interactions often exist between these functions. For example, the biological function of providing energy that drives microbial activity in turn results in improved structural stability and creates organic material that contributes to cation exchange capacity and pH buffering. Organic matter fractions Soil organic matter is not uniform. Dr Baldock and colleagues have identified four biologically significant carbon fractions (Table 1, page 4) and have now proposed the contribution that organic matter and its component fractions make to several of these functions (Figure 1). The findings reported are from a study of 29 different soils, sampled between zero and 10 centimetres, from nine locations in south-eastern Australia. The total quantity of carbon was found to be predominantly influenced by the amount of plant continued page 4 The four main components of soil organic matter. Increasing width of each form within the shape indicates its relative importance.
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