Effect of organic carbon on available water in soil

A model of the water-holding characteristics of soil is needed to develop a systematic method for determining the value of organic C in soil. In the United States, available water-holding capacity (AWC) in soil is that water retained in soil between field capacity and the permanent wilting point; these limits are approximated by that water retained between two energy limits: −1500 matrix potential (hygroscopic and micropore water) and about −33 kPa matrix potential (capillary rise). The General Energy Model for Limited Systems (GEMLS) was used to describe the effects of clay, silt, and organic matter on the AWC limits. The U. S. national soil inventory database (more than 100,000 entries) was segmented into narrow ranges of organic C content and silt content. The data from each subset were plotted as a function of soil clay content. Because of an apparent matrix transition effect, two complementary GEMLS functions were used to describe the −33 kPa and −1500 kPa water content as a function of soil clay, silt, and organic C contents. The model used six parameters (two function coefficients, two energy coefficients, and two critical clay contents), and required an initial manual fit of the models to the data subsets (about 100 ± 20 observations). Criteria for acceptance were uniform and homogenous distribution of the model residuals, absence of a detectable trend in the residual distribution, zero error sum, and maximal R. The primary energy coefficients were correlated with silt content. After the initial manual fit, the data were subjected to analysis using the SAS PROC MODEL procedure and a variable energy coefficient. Subsequent analyses indicated a complex relationship between the energy coefficients and the soil organic C content. A 1% increase in soil organic carbon causes a 2 to >5% increase in soil AWC depending on the soil texture.