Variation of Soil Morphological Properties Within Miami, Celina, and Crosby Mapping Units in West-Central Ohio 1

Variation of soil morphological properties within mapping units of Miami, Celina, and Crosby soils in Ohio has been statistically summarized. Ten randomly selected profiles within each of 24 mapping delineations of these soils were sampled for this characterization. The most variable properties were horizon thickness, depth of leaching of carbonates, loess thickness, depth to mottling, pH, and class (size) of soil structure. Clay content, grade (strength) of soil structure, and soil color were least variable. The number of observations required to estimate the population mean of the above parameters within certain limits using a .95 confidence interval was computed. These soils were correctly classified with regard to great group at 96% of the 240 observations, to subgroup at 85%, to soil series at 42%, and to soil type at 39%. Parent material was mapped accurately 88% of the time; erosion, 94%; pH, 70%; solum thickness, 63%; and drainage class, 65%. Since all delineations contained 30% or more inclusions of other soils, these mapping units would be considered complex or undifferentiated units based on the present concept of a mapping unit. It is proposed that the concept of the mapping unit be modified to emphasize the dominant soil of the area rather than implying 85% mapping accuracy of a specified soil. T HE INCREASING NUMBER of objectives which soil surveys now serve in both urban and rural areas, requires that greater emphasis be placed on more accurate and quantitative definitions of the basic interpretative element of surveys, namely the mapping unit. Although a number of studies have been conducted to evaluate physical and chemical variability within a taxonomic unit such as soil series (3, 4, 7, 9, 11) surprisingly little information has been accumulated or published to document variability within a cartographic unit, a mapping unit denned in terms of one or more taxonomic units. Two recent papers (1, 5) suggest that soil variability within mapping delineations is a serious problem in soil site studies and interpretations of soil physical and chemical properties. The purposes of this paper are: (i) to present central tendency and variance statistics for soil morphological properties observed within mapping units of Miami, Celina, and Crosby soils of west-central Ohio (Fig. 1); and (ii) to report the character and magnitude of soil inclusions occurring within each of these mapping units. Mapping units of each Miami catena member are denoted as "60" or "6A". Those units representing current concepts of Miami, Celina, and Crosby series are designated as 60. Those mapping units designated as 6A have tentatively been differentiated from 60 units on the basis of soils with somewhat finer-textured B horizons and thinner sola. In this study only silt loam soil types and slightly eroded phases on 2 to 6% slopes for Miami and Celina, and O to 2% slopes for Crosby soils were investigated. Throughout the remainder of the paper all mapping units will be designated simply in terms of their respective series taxonomic name, plus the 60 o>6A symbol without ;eferring to soil type, slope, or erosion. 712 SOIL SCIENCE SOCIETY PROCEEDINGS 1965