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

32MANAGING SOIL ORGANIC MATTER: A PRACTICAL GUIDEOrganic matter and cation exchangeThe ability of a soil to hold positively charged cationssuch as calcium, magnesium, potassium, sodium,hydrogen and aluminium at a given pH determinesits cation exchange capacity. Cation exchangemeasures the ability of a soil to hold on to and supplynutrients to plants. The cation exchange capacity ofa soil provides information on its structural stability,resilience, nutrient status and pH buffering capacity.Sodium and aluminium are negatively correlatedwith plant growth. Soil test results are expressedeither in milliequivalents per 100 grams soil(meq/100 g) or centimoles of charge per kilogram(cmol/kg).Soils have variable cation exchange capacityranging from sands, with a very low cation exchangecapacity often less than 3 meq/100 g, to vermiculite,which may hold up to 200 meq/100 g. Kaoliniticclays have a moderate cation exchange capacityof about 10 meq/100 g, while other clays such asillite and smectite have a higher exchange capacity(Purdie 1998).Table 4.1 contains information on the cationexchange of clay minerals.Table 4.1 Indicative cation exchange capacity ofdifferent clay minerals in soil (Moore et al. 1998).Clay mineralCation exchange capacity (meq/100g)Kaolinite 3-15Illite 10-40Montmorillonite 70-100Smectite 80-150Vermiculite 100-150Humified organic matter has a very high cationexchange capacity from 250-400 meq/100g. Therefore, in soils with low clay content theamount of humus and resistant soil organic matteris increasingly important to nutrient exchangebecause its large surface area gathers (adsorbs)cations from the soil solution, holding nutrientsthat would otherwise leach. Williams and Donald(1957) estimate that each percentage increase insoil organic carbon is the equivalent of 2.2 meq/100g cation exchange and in some soils contributesas much as 85 per cent of the cation exchangecapacity (Helling et al. 1964; Turpault et al. 2005;Hoyle et al. 2011).The contribution of organic matter to soil cationexchange capacity declines with soil depth, decreasingsoil pH (i.e. increasing soil acidity) and with increasingclay content (see Table 4.2).Table 4.2 Indicative cation exchange capacity fordifferent soil textures and organic matter.Soil textureCation exchange capacity (meq/100g)Sand 1-5Sandy loam 2-15Silt loam 10-25Clay loam/silty clay loam 15-35Clay 25-150Organic matter 40-200Humified organic matter 250-400NITROGEN, PHOSPHORUS, SULPHURAND ORGANIC MATTEROrganic matter contains a large store of nutrients— the majority of which are unavailable for plantuptake. It estimated that 2-4 per cent of soil organicmatter is decomposed each year (Rice 2002). Usingan average three per cent turnover and based on acarbon to nutrient ratio of 1000 (C):100 (N):15 (P):15(S), this suggests for a soil which has 1400 tonnesof soil per hectare and a soil organic carbon contentof 2.1 per cent, there would be a release of about 88kg nitrogen, 13 kg phosphorous and 13 kg sulphureach year from organic matter.Nitrogen supplyIn most soils, while nearly all nitrogen is presentin organic form, plants are generally better able totake up inorganic (mineral) nitrogen forms such asammonium (NH 4+) and nitrate (NO 3-). Nitrate is thedominant form of nitrogen taken up by agriculturalplants.The conversion of organic nitrogen to inorganicnitrogen is a biological process associated withthe mineralisation (decomposition) of organicmatter. Mineralisation results in the productionof ammonium, which is predominantly takenupby and immobilised within soil microbes andthen transformed via nitrification to nitrate. Theseprocesses can be limited by soil pH Caless than 5.5,poor soil permeability resulting in water-logged soils,carbon availability, drying soils and temperaturesbelow 20°C (Mengel and Kirkby 1987).Soil biological processes are also integral to the