Evaluating central tendency and variance of soil properties within map units

Users are demanding that soil surveys contain statistical information about soil and map unit properties. Numerous sampling and statistical methodolog ies are available, but statistical methods should be appropriate for both s ampling methods and data distributions. Rigorous investigations of soil pro perty distributions are rare, and statistical methodologies have received l ittle attention for soil survey applications. Two hypotheses were tested: ( i) frequency distributions of soil properties within map units are non-norm ally distributed, and (ii) variability estimates will depend on the statist ical assumptions and methods. Twelve transects were randomly selected in a Boone County, Missouri soil survey map unit and 12 soil attributes were inv estigated from a population of 120 soil cores. Three methods were used to e stimate variances: (i) simple random sampling, (ii) cluster sampling, and ( ii) two-stage random sampling. Frequency distributions were significantly s kewed for 11 of the soil properties. The median is an appropriate estimator of central tendency for these properties. The random sampling approach pro vided the smallest variances, but is conceptually questionable. Two-stage s ampling is conceptually most correct, but requires dubious assumptions abou t population size, Nonparametric confidence intervals for variable medians were appropriate, but are wide for most properties. None of the applied ass umptions and techniques are universally applicable. Statistical assumptions and methodologies should be tested across a spectrum of depositional, weat hering, and land-use conditions to determine the most suitable approaches f or particular user needs and soil conditions.