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

mycorrhizal associations have been known to causeyield penalties (Ryan et al. 2002, 2005).Arbuscular mycorrhizal fungi have a symbiotic(defined in the broadest terms as two ormore organisms living together) associationwith the root of a living plant and are primarilyresponsible for nutrient transfer.Organic phosphorus sources are relatively slowrelease,but it is not easy to predict exactly whensoluble phosphorus will become available. Toensure a more reliable supply of phosphorus it isbest to apply a combination of mineral and organicsources. Soils with a high tendency to adsorb orfix phosphorus are likely to require phosphorusfertiliser application to meet crop requirementseven when soil tests suggest sufficient levels ofphosphorus exist.Diffusive Gradient in Thin-Films (DGT) is a newmethod of measuring soil phosphorus (Mason et al.2010) and is shown to be more accurate than otherconventional methods in estimating the phosphorusrequirement of crops. It provides an improvedmeasure of the phosphorus available for plantuptake and determines the likely yield responsefrom additional fertiliser (see http://soilquality.org.au/factsheets/dgt-phosphorus for further information).SULPHURSulphur is essential for plant protein production andsulphur deficiency can lower grain quality. Sulphur isalso critical for effective nitrogen fixation in legumes.Cereals typically require twice the amount of sulphuras phosphorus. As sulphur is relatively immobile inplants, a sustained supply of the mineral is requiredfrom the soil.Sulphur and organic matterMost sulphur in soil is bound in soil organic matterfor surface soils (0-10 cm). The ratio of carbon tonitrogen to phosphorous to sulphur in soil organicmatter is usually about 108:8:1:1 (108 units ofcarbon, eight units of nitrogen and one unit ofphosphorous and sulphur). Many soils also containgypsum in their subsoil layers.Sulphate (SO 4-2) is mineralised when soil organicmatter is broken down and is the most plant-availableform of sulphur in well aerated soils. Sulphate ismade available to plants from organic matter with acarbon to sulphur ratio of less than 200:1. Residueswith a carbon to sulphur ratio of more than 400:1usually result in sulphur immobilisation (Delgadoand Follet 2002). Sulphate not taken up by plants isvulnerable to leaching and in coarse textured soilsunder high rainfall sulphur is often deficient.Sulphur deficiencies in the soilHistorically, sulphur deficiency has been rarein Australia because of the widespread use ofsuperphosphate fertilisers containing sulphur.However, in recent years sulphur deficiency hasbecome more evident with the switch to low-sulphatephosphate fertilisers and the increasing adoptionof canola, which has a high sulphur requirement.Sulphur deficiency is particularly apparent in soilswith high nitrogen availability, which increases yieldpotential and therefore sulphur demand. Removalof sulphur in organic matter via the harvest of cropsand pastures can be rectified with a replacementstrategy of between 2-5 kg of sulphur per tonne ofgrain or biomass removed (or in the case of canola10 kg of sulphur per tonne of removed grain orbiomass).POTASSIUMBetween 95-98 per cent of the potassium in soilis unavailable to plants and exists as a structuralcomponent of soil minerals until broken downby weathering processes. Instead, plants largelyacquire potassium in the form of exchangeablepotassium, or dissolved potassium available in soilsolution. Available soil potassium results from thenet effects of supply processes, including mineralweathering, addition of fertilisers and mineralisationof organic inputs against losses associated withleaching, erosion, plant uptake and fixation.Soil organic matter increases the soil’s cationexchange capacity and in doing so increasesthe amount of soluble potassium, calcium andmagnesium available for release during mineralisation(Delgado and Follet 2002). Potassium is alsoreleased relatively quickly from crop residues tocontribute to the non-exchangeable, exchangeableand soil solution potassium pools. However, unlikenitrogen and phosphorous, available potassium inmany situations appears more closely linked to soiltype (clay complex) than soil organic matter.37MANAGING SOIL ORGANIC MATTER: A PRACTICAL GUIDE