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

43Reported changes in soil water content due toincreases in organic matter often relate to changesin water holding capacity and are not necessarilyindicative of an increase in plant-available water. Thedifference between soil water holding capacity andplant-available water can vary widely depending onthe mineral composition of the soil, the form andlevel of organic matter and whether associatedchanges in bulk density have been considered. Ingeneral, increasing soil organic matter will have asmaller effect on plant available water in soils withincreasing clay content. It is therefore critical to takeall these factors into account when assessing thepotential impact of soil organic matter on soil watercontent.HOW IMPORTANT IS ‘EXTRA’ WATER TOAGRICULTURAL PRODUCTIONThe value of ‘extra’ soil water storage to crop andpasture yield depends on the amount and frequencyof rainfall or irrigation as well as the water demandof the crop or pasture. If the water is available at atime when crops are otherwise water stressed, evena small amount of water could lift crop or pastureproductivity. As a rule, each additional millimetreof rainfall (where water limited) can produce anadditional 20 kg grain per hectare of wheat (Frenchand Schulz 1984). However, where there is sufficientwater availability or when soil constraints preventplants accessing water then any extra soil moisturemay not be used.RELATIONSHIP BETWEEN ORGANICMATTER AND WATER REPELLENCEOrganic matter is not always associated withimproved water holding capacity because organiccompounds that exhibit water repellent properties(see Plate 5.3 a) such as plant waxes can beimplicated in the development of non-wetting soils(see Plate 5.3 b).Although not restricted to any one climate or soiltype, water repellence most often affects sands.An estimated 2-5 million hectares of agriculturalland in southern Australia exhibits signs of soilwater repellence (Roper 2004), with coastal sandplainsoils especially prone. Water repellence is asignificant issue in agricultural systems becauseit slows water infiltration and constrains soil waterstorage. This often results in poor or uneven cropestablishment and development, variable weedcontrol, increased risk of soil erosion and decreasedgrain yield. Nationally, estimated losses for crop andpasture production due to water repellent soils isabout $100-$250 million per year.An easy to use web-based tool for adjustingyour soil carbon and nutrient concentrationresults for either bulk density or gravel/stonecontent is available at www.soilquality.org.au/calculators/gravel_bulk_densityWHAT CAUSES WATER REPELLENCE?While all plants have waxes to prevent theirdesiccation, a range of more than 50 plant speciesincluding eucalyptus, acacia, clover and lupin andvarious organic compounds, including fungal derivedsources, have been more closely associated withMANAGING SOIL ORGANIC MATTER: A PRACTICAL GUIDE