Distribution and Variability of Surface Soil Properties at a Regional Scale

Information on the probability distribution and variability of soil properties at a regional scale could improve the ability of the USDA-Natural Resources Conservation Service (NRCS) to monitor soil condition using the National Resources Inventory (NRI). Our objective was to evaluate the hypothesis that the probability distribution of 17 physical, chemical, and biological soil properties are: (i) normally distributed, or (ii) log-normally distributed at a regional scale, and to estimate the magnitude of change that may be detected assuming either a normal or log-normal distribution. Samples were collected irrespective of soil series from two Major Land Resource Areas (MLRAs) (no. 9 and 105), and from the Ascalon (fine-loamy, mixed, superactive, mesic Aridic Argiustoll) and Amarillo (fine-loamy, mixed, superactive, thermie Aridic Paleustalf) soils in MLRA 67 and 77, using the NRI sampling design. Most soil properties were non-normally distributed, with the frequency of non-normality varying between MLRAs. Confining sampling to a single soil series did not consistently improve the precision with which soil properties were estimated. Log e transformation resulted in normal distributions for most soil properties and reduced variability two- to threefold. However, a few soil properties remained non-normally distributed. Soil pH may be monitored at the regional scale with a high degree of precision. Small changes in soil C content (3-8% of the regional mean) may be detected using log, transformed total organic C as the indicator. Sampling soil properties as part of the NRI should improve NRCS' ability to monitor soil condition on a regional scale.