A generalizable index of soil development

The growing influence of climatic and land-use pressures on soils across broad geographic extents have revealed a need for generalizable indices of soil development applicable at continental scales. In this study, we present generalizable horizon and profile development indices (HDIs and PDIs, respectively) based primarily on relative horizon properties instead of parent material information and provide an evaluation of these indices at a continental scale. The indices developed in this work combine knowledge of pedogenic processes with selected properties from a large database of predominantly US soils to yield relevant soil morphological, physical, and chemical information on 57,171 horizons arising from 8980 pedons. We selected four properties from which the generalizable HDI and PDI calculations were based: particle size, calcium carbonate equivalent (CCE), color, and clay films. Modified z-scores were used to assess relative changes between either eluvial and illuvial horizons for particle size and CCE or surface and subsurface horizons for color. These values were then normalized using maximum and minimum values observed in the dataset. Normalized clay film scores were calculated using percent cover along with ordinal values assigned to distinctness categories described in the field. The index values from HDI compared well to both diagnostic horizon designations and chemical index of alteration and PDI values matched the sequence of taxonomic soil orders that represent a development gradient. In addition, we observed a correspondence between calculated PDIs and a clay mineralogy index representing weathering intensity. Relationships between these generalizable indices, evaluation metrics, and climate (i.e., MAP) explored in this work opens the door to numerous broad-scale applications in the future such as pedogenic modeling, identification of soil anomalies, and estimation of surface soil ages.