The study of pedodiversity, and in particular soil richness, depends on the notion of soils as discrete entities. Soil classification systems are often criticized in this regard because they depend in part on arbitrary or subjective quantitative criteria to subdivide and classify soils. In this study soils were categorized on the basis of the presence or absence of six lithological and morphological characteristics, with no subjective criteria or imposed thresholds or subdivisions. Richness vs. area relationships, and the general pattern of soil variability and diversity, were then compared to previously-published analyses of pedodiversity based on soil taxonomy.

The study area consists of 16 0.13-ha plots on forested sideslopes of the Ouachita Mountains, Arkansas, with a minimum of 20 classified soil pits per plot. Soils have been classified and mapped in previous work according to U.S. Soil Taxonomy. In this study an ad hoc classification was developed, from the standpoint of soil geomorphology and studies of the coevolution of soils and landscapes, and based on the regional environmental framework. Soils were classified based on (1) the underlying geology (shale, sandstone bedrock, or transported sandstone rock fragments), and on the presence or absence of (2) texture-contrast subsoils, (3) eluvial horizons, (4) surface and/or subsurface stone lines or zones, (5) lithological contrasts between soil and underlying geology, and (6) redoximorphic features within the solum.

The soil geomorphic classification yielded 40 different soil types (out of 288 possible different combinations of the criteria), compared to 19 different series or taxadjuncts indentified by standard soil classification. Individual plots contained five to 11 different soils, with extensive local variability. A standard power-function relationship between soil richness (S) and area or number of samples (A) provided the best fit for most plots (S = c A^b). The exponent b was slightly higher than for the taxonomy-based analysis, but in general the analyses lead to similar conclusions with respect to the relationship between richness and area, and the relative importance of local, within-plot versus regional, between-plot variability. Results support the view that soils can be viewed and treated as discrete entities, and that highly localized variability may be critical to pedodiversity.