Submitted to: Soil Science
Publication Type: Peer reviewed journal
Publication Acceptance Date: 10/7/2004
Publication Date: 2/1/2005
Citation: Olness, A.E., Archer, D.W. 2005. Effect of organic carbon on available water in soil. Soil Science. 170(2):90-101. Interpretive Summary: Soil water is a critical factor in crop yield. The importance of soil organic matter in determining the amount of water held by soil has been debated for a century. A new approach describes the role of soil organic matter in retaining water available for plants. The model describes how soil organic matter interacts with soil particle size to cause changes in the amounts of water held in the soil. The data used were collected over several years by the USDA-NRCS-Soil Survey. By using a recent theory, the data show that soil particle size has a huge influence on the benefits obtained from increasing soil organic matter. The results from this work show close agreement with several earlier authors. The results also unify the ideas developed over several years of work. The model developed enables a determination of the value of increasing soil organic matter in terms of crop production. By using the theories presented, agronomists and economists can estimate an increase in crop yield due to carbon and indirectly determine the value of soil organic matter. The resulting valuations will give policy makers and legislators guidance in making effective agricultural programs with soil conservation.
Technical Abstract: A model of the water-holding characteristics of soil is needed to develop a systematic method for valuing organic C in soil. Water is held in soil between two energy limits: hygroscopic water (-1500 kPa) and capillary rise (about -33 kPa). The General Energy Model for Limited Systems (GEMLS) was used to describe the effects of clay, silt and organic matter on the available water limits. The US 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**2. The energy coefficient was correlated with silt content. After the initial manual fit, the data were subjected to analysis using the SAS PROC MODEL and a variable energy coefficient. Subsequent analyses indicated a complex relationship between the energy coefficient and the soil organic C content. A 1% increase in soil organic carbon causes a greater than 2% increase in soil available water content depending on the soil texture.