Available Water Capacity and Soil Organic Matter

Abstract

Available water capacity (AWC) is defined as the amount of water (cm water=100 cm soil) retained in the soil between the ‘‘field capacity’’ (FC) and the ‘‘permanent wilting point’’ (PWP). Field capacity and permanent wilting point are defined as the volumetric fraction of water in the soil at soil water potentials of 10–33 and 1500 kPa, respectively. One of the paradigms of soil science is that AWC is positively related to soil organic matter (SOM) because SOM raises FC more than PWP matter enhances soil water retention because of its hydrophilic nature and its positive influence on soil structure. Increasing SOM increases soil aggregate formation and aggregate stability increasing porosity in the range of pore sizes that retain plant-available water and enhancing infiltration and water retention throughout the rooting zone. When SOM decreases, soil aggregation and aggregate stability decrease and bulk density increases. These changes in the physical properties result in lower infiltration rates and higher susceptibility to erosion. This article reviews literature on the sensitivity of AWC to SOC and discusses the environmental implications of changes in SOM and AWC.