Determining the Least Limiting Water Range Using Limited Soil Data.

The Least Limiting Water Range (LLWR) is a useful tool to evaluate changes in soil physical condition caused by changing soil management. It incorporates limitations to plant growth based on limiting aeration, water holding capacity and soil strength. A disadvantage of the LLWR is the need to determine the water content at field capacity and wilting point and the water content – bulk density combination that determines limitations of soil strength. A simple exponential equation was used relating water content to pressure potential. Coefficients for the equation were calculated from known values of liquid limit of soil, soil bulk density, soil organic matter content (SOM) and previous compaction level. Once the water content at a specific pressure potential was determined, the degree of water saturation for a specified bulk density was determined. The effective stress was estimated from degree of water saturation and pressure potential. Soil penetrometer resistance was then estimated using the effective stress and bulk density. The verification of these methods was determined using independent data sets collected at the Central Great Plains Research Station in Akron, CO. The methods were somewhat insensitive to previous compaction level, but were very sensitive to quantification of liquid limit, bulk density, and SOM. Using these techniques the change in LLWR caused by tillage-induced changes in SOM of a Duroc loam were estimated. Lower SOM caused by plowing native sod caused a smaller LLWR, particularly at low bulk density. These techniques may be useful to quantify the effects of soil management on potential soil productivity.