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Frequency Distributions of Soil Properties on a Loess-mantled Missouri Watershed

Soil Survey, Lincoln Univ., 830 Chestnut St., Jefferson City, MO 65101

R. D. Hammer^* and D. Larsen

School of Natural Resources, Univ. of Missouri-Columbia, Columbia, MO 65211

*Corresponding author (hammerr{at}missouri.edu).

ABSTRACT

Frequency distributions of soil property values are largely unknown, yet they are important to studies of soil variability. Our objectives in this paper were to (i) evaluate the distributional normality of soil properties within and among upland geomorphic surfaces, and (ii) interpret these distributions in terms of pedogenic factors and processes. Sixty soil properties were measured from 257 pedons along point transects in a 40-ha upland Missouri pasture. Five of the 60 tested variables approximated normality. About three-quarters of the distributions were asymmetric (skewed), and over four-fifths were misshapen (kurtotic). Partitioning the samples by geomorphic surface (ridge, shoulder and backslope) improved the normalities, symmetries and shapes of many, but not all, distributions. A priori assumptions of normality are not warranted for most soil properties. Care should be taken when applying statistical tests that are sensitive to departures from normality. However, asymmetric or misshapen distributions provide meaningful information. Extended tails in skewed distributions represent outliers or inclusions in soil survey map units. These distributions result from variation in depositional environments or the asymmetric effects of pedogenic or hydrologic processes. Kurtotic distributions may indicate the presence within the sample of subpopulations with differing means or variances. Distributions provide important clues to understanding spatial variability.

Contribution from the Missouri Agric. Exp. Stn. Journal Series 12,515.

Received for publication August 27, 1996.

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