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Variability in Family Differentia of Soils in a Second-order Soil Survey Mapping Unit¹

ABSTRACT

Soil variability and its relation to family taxa used to classify the soils in a consociation used as a mapping unit in a second-order soil survey are evaluated statistically. Most of the variability is within delineations and results from short-range variability within and longrange variability among pedons. The ranges for the clayey particle size and thermic temperature classes correspond to those in this mapping unit. Ranges in soil mineralogy cannot be delineated by soil mapping since differences cannot be quantified. The use of mixed vs. oxidic mineralogy classes, as differentiating criteria at the family level for classifying soil series in this consociation, is questionable with the present definition of these mineralogical classes, since in some instances, both classes occur with 7 m.

Confidence intervals on the means, estimations of the means, and prediction intervals for future observations indicate that the number of samples needed to establish statistical validity in soil classification and correlation is much larger than presently collected.

The amount of variability in soils within 7 m and in sampling and analytical procedures is indicated by the percentages of the total variance contributed by the error terms in analysis of variance. These percentages are low for all the soil components used to classify the soils except hydroxy-interlayered vermiculite.

Key Words: pedon • soil taxonomy • soil classification • particle-size class • mineralogy class • landscape element

¹ Contribution from the departments of Agron. and Statistics, Virginia Polytechnic Inst. and State Univ., Blacksburg, VA 24061.

² Instructor, Research Associate, and Associate Professor of Agron.; Associate Professor of Statistics; and Research Associate of Agron., respectively.

Received for publication November 25, 1980. Accepted for publication August 26, 1981.