The Role of Increasing Soil Organic Matter in Enhancing Plant Available Water.

Historically, there has been conflicting data on the relationship between soil organic matter and plant available water.  Many early studies utilized mixtures of soil or sand with organic amendments in laboratory or greenhouse settings.  The majority of these studies showed inconsistent or non-significant improvement in stored water.  These results were likely due to the choice of amendment and the short-term nature of the measurements.  There is now considerable interest in whether soils of similar texture but varying organic matter content can exhibit varying water holding capacity.  If the relationship between organic matter content and plant available water can be predicted and the mechanisms understood, then the productivity of degraded, low organic matter content soils could be restored by practices that lead to organic matter accumulation.  The changing views on this topic with special attention to how the different conclusions may have been reached will be discussed.  Due to recent droughts, the current research focus is on quantifying the effect of increasing soil organic matter content and it's role in water retention.  It is also likely that, as crop yields and commensurate water use increase to meet global food demand, greater attention will be given to increasing the ability of the soil profile to store more water in rainfed systems.   Organic systems offer opportunities and challenges for increasing soil organic matter.  While organic-rich amendments (compost, manure, and waste materials) are commonly utilized in many systems, frequent tillage may result in lower-than-expected gains in soil organic matter content.  Other issues include the hydrophobicity of some organic amendements and the interaction between structure and soil organic matter related to moisture retention.