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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.
15MANAGING SOIL ORGANIC MATTER: A PRACTICAL GUIDEHow do I calculate the impact of incorporating organic residues on my soil nitrogen supply?ScenarioA grain grower retains three tonnes per hectarewheat stubble, with a C:N ratio of 120:1 andwants to estimate the likely impact of the stubbleon soil nitrogen levels in the paddock.Step IThe amount of carbon present in the plantresidues added to the soil.3000 kg of stubble x 0.45 carbon content =1350 kg of carbon in plant residuesStep IIThe amount of nitrogen present in the organicmatter added to the soil. The stubble contains1350 kg of carbon and has a C:N ratio of 120:1.1350 kg carbon ÷ 120 = 11.25 kg nitrogen inorganic matterStep IIIAllow for 30 per cent of the carbon being usedby microbes to grow, with the remaining 70 percent respired as carbon dioxide.The amount of carbon used by microbes is 1350kg carbon x 0.3 = 405 kg carbonGiven that microbes have a C:N ratio of 12:1they therefore require 1 kg of nitrogen for every12 kg of carbon to grow.405 kg carbon ÷ 12 = 34 kg NStep IVCompare the two nitrogen values; the freshorganic matter contained 11 kg of nitrogen andthe microbes require 34 kg of nitrogen to grow.The nitrogen balance = 11 – 34= - 23 kg N haA NEGATIVE balance indicates that nitrogenin the organic matter was LESS than thenitrogen required by the microbes. This nitrogendeficit will be sourced from the soil, making itunavailable for plants. In this case, a growershould consider fertiliser strategies which willensure sufficient nitrogen is available to plantsearly in their growth.If the nitrogen balance had been POSITIVEa surplus of nitrogen would then becomeavailable to plants because there would beMORE nitrogen than required for microbial use.If this was the case, growers may consider splitapplications of nitrogen to save input costs andminimise losses of nitrogen via leaching.Knowing the contribution of organic matterto your soils nutrition can help inform moreprofitable fertiliser management strategies.
Page 1 and 2: MANAGING SOILORGANIC MATTERA PRACTI
Page 3 and 4: FOREWORD1The one constant in agricu
Page 6 and 7: TABLES, FIGURES AND PLATES4MANAGING
Page 8 and 9: 6INTRODUCTIONMANAGING SOIL ORGANIC
Page 10 and 11: 018SOIL ORGANIC MATTERMANAGING SOIL
Page 12 and 13: Table 1.1 Size, composition, turnov
Page 14 and 15: Table 1.2 Functional role of soil o
Page 18 and 19: 16HOW MUCH ORGANIC CARBONREMAINS IN
Page 20 and 21: 0218BIOLOGICAL TURNOVEROF ORGANIC M
Page 22 and 23: 20MANAGING SOIL ORGANIC MATTER: A P
Page 26 and 27: 2403MANAGING SOIL ORGANIC MATTER: A
Page 29: less than 15 per cent of carbon inp
Page 36 and 37: Figure 4.1 Nitrogen cycling in soil
Page 39: mycorrhizal associations have been
Page 42 and 43: Table 5.1 Influence of soil charact
Page 44 and 45: Figure 5.2 Adjustment of organic ca
Page 48 and 49: 0646ORGANIC MATTER LOSSESFROM SOILM

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