Submitted to: Natural Organic Matter in Soils and Water North Central Region Symposium
Publication Type: Abstract Only
Publication Acceptance Date: 7/19/2002
Publication Date: 7/19/2002
Citation: OLNESS, A.E., ARCHER, D.W. USING GEMLS TO DESCRIBE THE CONTRIBUTION OF ORGANIC CARBON TO SOIL WATER HOLDING CAPACITY. PROCEEDINGS OF NATURAL ORGANIC MATTER IN SOILS AND WATER NORTH CENTRAL REGION SYMPOISUM. 2002. P. 17. Interpretive Summary:
Technical Abstract: A detailed descriptive model of the water holding characteristics of soil is needed to enhance the USDA-ARS N fertilizer decision aid and develop a systematic method for valuing organic C in soil. The General Energy Model for Limited Systems (GEMLS) was evaluated for this purpose by applying it to the U.S. national soil inventory database (more than 100,000 entries). The database 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 a 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. This entailed 6 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 trendline in the residual distribution, zero error sum, and maximal R**2. The energy coefficient was a near linear function of silt content. After the initial manual fit, the data were subjected to analysis using SAS PROC MODEL and a variable energy coefficient. This work showed that the energy coefficient was also a complex function of organic C content. The two function coefficients appear to be constants and the two critical clay content values were linearly related to silt content. The R**2 values for C contents < 2 % often exceeded 0.9. The final product is a continuous function capable of predicting the water holding content of soil as a function of its physical separates.