Linking process capability analysis and least limiting water range for assessing soil physical quality

In the process of evaluating the physical quality of soil for crop production, measurable sources of stress that the soil imposes on growing crops must be identified. Approaches for monitoring or evaluating soil physical quality should then be based on properties or processes that relate to these stresses and must be measured against definable standards. We hypothesized that process capability analysis applied to measurements of soil water content and the least limiting water range (LLWR) would meet these requirements and could be used to evaluate the physical quality of soils for crop growth. Previously published data obtained over 3 years in a field with a variable landscape planted to corn under no-till were used to test the hypothesis. The temporal variability of soil water content was regarded as a process which aims to generate individual values for soil water content inside the limits specified by the LLWR. Process capability analysis successfully linked the temporal variability of soil water content in relation to the LLWR. The main process capability parameter, i.e. distance to nearest specification (DNS) varied by a factor of three across the landscape and was related to clay and organic carbon contents. Values of DNS were strongly correlated with shoot growth (R² = 0.97) suggesting that DNS effectively characterized the spatial variability in stresses imposed on plant growth by soil and described changes in the soil physical quality for crop growth across the site. The results supported our hypothesis.