Organic Matter and Soil Organic Matter Contribution to Available Water-Holding Capacity – How Much and What It Means for Water Conservation Practices.

Soil organic matter (SOM) serves several key edaphic functions that includes contributions to available water-holding capacity (AWHC), which has practical implications for soil water management, especially for agricultural purposes. Recent embellished estimates on the contribution of SOM to AWHC have raised the question about the magnitude of this contribution. The magnitude of AWHC due to SOM has practical implications for soil water conservation practices and predicted gains associated with increases in SOM. We evaluate the contribution of SOM to AWHC as influenced by soil physical properties (particle size, texture, bulk density) and mineralogy using the NCSS Soil Characterization Database and also by calculations based on fundamental properties (porosity and density) of SOM, Silt content had the greatest correlation with AWHC (r = 0.56). A decrease of soil bulk density (BD) increased AWHC (r = -0.34), but the relationship to BD was highly variable depending on SOM and soil texture. Thus, for samples between 0-8% SOM a weak correlation with AWHC (r = 0.27) was observed, but a moderate correlation (r = 0.62) was observed for the range between 0 and100% SOM. The increase of AWHC was more pronounced for sandy than for silty clay loam and silt loam textures. Soils with clay contents greater than 40%, had variable correlation coefficients depending on minerology class: mixed (r = 0.24), smectitic (r = 0.08), and kaolinitic (r = 0.49). The empirical data showed that overall, a 1% increase in SOM content increased AWHC, on average, a magnitude of about 1.5% times its weight, depending on soil texture and clay mineralogy. These values were consistent with the theoretical calculations that showed the potential AWHC increase (on a volumetric basis) with a unit increase in OM (wt. %) is about 1.5 to 1.7% for the 0-8% OM range, common to most agricultural soils. This equates to 10,800 liters (2,800 gallons) of water for each additional 1% increase in SOM (up to 8% OM) for a layer thickness of 15 cm covering 0.4-ha area (an acre furrow slice). These results support other findings that OM alone may hold up to twice as much water per unit volume as mineral soils, but not all in the AWHC range. Furthermore, when decomposed and incorporated with the mineral soil fraction the contribution SOM makes to AWHC is complex and generally positive, but in some soils an increase in SOM decreases AWHC.