Using Multi-Scale Sampling Requirements Tool (MSSRET) for Sampling Sufficiency Analyses of a Southern Utah Project.

The Multi-Scale Sampling Requirements Tool (MSSRET) is an Excel based tool that can be used for project planning and sampling sufficiency analysis. It reports the number of samples needed to detect a minimum detectable difference (MDD) or the actual MDD of a finished project. The purpose of this project was to analyze MSSRET's utility for a comparison study of dynamic soil properties on the Begay fine-sandy loam, 0 – 6 % slope, soil map unit component phase in southern Utah. This soil is part of the Semidesert Sandy Loam (Fourwing Saltbrush) ecological site. Two state phases, or vegetative communities, representing two different ecological states, were compared: perennial grass-shrub (PGS) and annual cheat-grass (AG). Four 20m² plots were located randomly within each state phase of the map unit component with 6 PGS and 4 AG sample locations within each plot. The actual MDD was smaller than the desired MDD (set to 20% of the overall mean) for bulk density at all depths; meaning the samples and plots in this project were adequate to detect a 20% difference in state phase bulk densities. Sampling was not adequate to detect a 20% difference in soil organic carbon (SOC) at the surface (0 – 2 cm). As many as 28 plots (maintaining 4 and 6 samples per plot) would be needed to detect a 20% difference in % SOC. This is due both to the small mean (20%-MDD of SOC is 0.13%) and high relative variances of organic carbon in these state phases. Using carbon density (organic carbon on a per volume basis) greatly decreased both residual and plot variance, leading to reduced plot requirements (2 plots per state phase with 4 and 6 samples). This study illustrates how the MSSRET tool (available at http://soils.usda.gov/technical/soil_change/index.html) can be used to develop more efficient sampling designs.