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

Soil organic matter and in particular the humusfraction can hold several times its own weight inwater. It seems logical, then, that increasing theorganic matter content of soil would have a positiveimpact on the water holding capacity of a soil.However, while there is indeed an establishedlink between soil organic matter and water holdingcapacity, its importance declines with soil depth andincreasing clay content (Hoyle et al. 2011). About60 per cent of the organic matter in the top 30 cmof soil occurs in the surface layer (0-10 cm), so theinfluence of soil organic matter on soil water is mostevident in the topsoil.There is little influence of organic matter on plantavailable water late in the season when soil moistureis usually below 30 cm. Clay also functions toabsorb soil water, decreasing the relative influenceof soil organic matter on water holding capacity asthe clay content of a soil increases (see Figure 5.1). Figure 5.1 Change in water holding capacity forthe 0-10 cm soil layer of South Australian redbrownearths, with a one per cent increase in soilorganic carbon content (Hoyle et al. 2011).The relative value of available soil water dependson the amount and frequency of rainfall in any oneseason. For example, any additional water holdingcapacity provided by soil organic matter is only likelyto be beneficial when intermittent rainfall results inlow, infrequent or variable soil water. In contrast,frequent or high in-crop rainfall is likely to diminish theimpact of soil organic matter on soil water holdingcapacity. In addition, and depending on soil type,the amount of extra water held does not necessarilymean it will be available to plants due to its locationin the profile relative to roots, or because it is heldso tightly by the soil that it cannot be extracted bythe plant.Maximum amount of extra water stored froma 1% increase in soil organic carbon (SOC)1% SOC = 14 t C ha* = 5.6 mm*** Calculated to a depth of 10 cm in soil with abulk density of 1.4** Actual plant available water will be influencedby soil texture** Assumes soil organic matter holds four timesits weight in waterWHAT AFFECTS THE WATER HOLDINGCAPACITY OF A SOIL?The texture and structure of a soil determine its abilityto form soil aggregates, which help determine waterand nutrient storage (see Table 5.1). A soil organiccarbon content of two per cent is considered optimalfor aggregate stability (Kay and Angers 1999).The spaces or pores that form between soilaggregates are capable of storing water andprovide a home for soil biota. Soils with a widerange of particle sizes are unstable and are easilycompacted to form dense, often impenetrablelayers that constrain root growth. Organic matterhelps to create and stabilise soil pores, promotesthe formation of soil aggregates and contributes tosoil water storage via its capacity to absorb water.BARRIERS TO ROOT GROWTHPhysical, chemical and biological subsoil constraintscan prevent plant roots from growing to depth.Pathogens, physical impediments or chemicalbarriers such as subsoil acidity and salinity can slowplant root growth and prevent access to subsoilmoisture. Improving soil structure and removingthese barriers to plant growth can improve thewater storage capacity of the soil and increase thearea and depth of soil available to plant roots forexploration.Water and nutrients can move through compactedsoil (see Plate 5.1), but often remain unavailable toplants because of restricted root growth. When soilstrength measures about 2000 kilopascal (kPa), orhas a bulk density higher than 1.7 g/cm 3 root growthtypically stops, continues horizontally or occurs at aslower rate. Bulk density is influenced by soil organic39MANAGING SOIL ORGANIC MATTER: A PRACTICAL GUIDE