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

Table 3.1 The influence of soil organic matter on soil attributes and functions.Soil attribute or function Mode Impact Quantification29Soil architectureSoil fertilityWater infiltrationMicrobial by-products such asbacterial glues and fungal hyphaecan increase soil aggregation andstability of soil structure.Decomposition of organic matterdetermines nutrient supply.Microbial by-products suchas bacterial glues and fungalhyphae increase potential for soilaggregation.Increasing organic matter can beassociated with development ofwater repellence on soil with lessthan 15 per cent clay.Can improve water infiltration,soil porosity and the exchange ofwater and oxygen.Increased plant productivity.Decreased input costs.Decreases moisture loss fromrun-off, evaporation.More water enters the soil profile.Increased soil porosity helps theexchange of water and oxygen.Minimal impact on coarsetextured sands.Up to 80 per cent of nitrogenuptake results from the turnoverof organic matter.Soils can supply between 50-100kg nitrogen per hectare eachyear.Organic matter also containsphosphorous and sulphur.Increasing the rate of entry willlead to a greater potential formore available water.MANAGING SOIL ORGANIC MATTER: A PRACTICAL GUIDEWater holding capacity (or‘bucket size’)Soil organic matter can holdseveral times its own weight inwater.Increased plant available waterand decreased deep drainagebelow the root zone.Supports higher productivity.A one per cent increase insoil organic carbon stores theequivalent of a maximum 5.6 mmof soil water in the top 10 cm.Depending on soil type only aproportion of this will be plantavailable.Soil biological processesOrganic matter is the primarysource of energy required by soilmicroorganisms for growth andreproduction.Influences nutrient cyclingand availability, soil diversity,resilience and impact of stressevents(i.e. pathogen impact, recovery).The microbial efficiencyinfluences what percentage oforganic matter is retained in soils.A greater proportion of carbon isretained in soils with increasingclay content.In Australia, typically less than30 per cent of carbon is retained(can be up to 50 per cent).Buffers soil pH(helps maintain acidity at aconstant level)Organic matter is alkaline in itsnature.Large amounts of residues or aconcentration of burnt residuescan increase soil pH.Burnt residues can have a limingeffect (e.g. often observed onburnt windrows) resulting inhigher yielding areas.Burning residues will result inless labile carbon entering thesoil.Soil resilience — defined asthe ability of a soil to recoverto its initial state after astress eventIncreasing organic mattersupports an increasing diversityof microorganisms, many ofwhich can undertake similar rolesallowing for some redundancy.Increased ability of soil to recoversoil function after a disturbance.Less than 10 per cent of thepopulation is active at any time.It is estimated that losses of upto 20 per cent of the microbialbiomass would have little impacton soil function.