QUANTITATIVE CHARACTERIZATION AND PEDOGENIC DEVELOPMENT OF SOIL STRUCTURE

Abstract

Soil particles are often arranged into repeating patterns of aggregates with similar shapes, sizes, and degrees of expression. These repeating aggregates, known as ‘peds,’ are currently described using qualitative and subjective categories for type, size, and grade as follows. Peds are assigned a type class (e.g., platy, granular, prismatic, etc.) based on overall ped shape. Peds are classified into size categories (e.g., fine, medium, and coarse) based on quantitative ped width and thickness criteria. Peds are assigned a grade class (e.g., weak, moderate, or strong) which describes the degree of expression. Soil structure develops as a result of complex interactions with climate, organisms, relief, parent material, and time. However, our understanding of these interactions is limited by the categorical and subjective nature of ped descriptions and the lack of datasets that include a wide range of variability in the factors responsible for the development of soil structure. Therefore, the first objective of this dissertation was to develop a method to quantify soil structure using morphometric indices for ped shape by analyzing previously published digital photographs of soil profiles and structural specimens. The second objective of this dissertation was to assemble an easily-accessible, two-dimensional data matrix containing laboratory and field-based measurements of soil properties across the USA and integrate topographic, climatological, and ecological data to, ultimately, explore the response of soil structure to exogenous and endogenous factors in both surface A horizons and subsurface B horizons. To those ends, we assembled two databases: the Ped Shape Digital Morphometric (PSDM) database and the University of Kansas Research Dataset of Soils (KURDS). The PSDM database was used to develop new morphometric indices of ped silhouettes quantitatively describing ped shape. These morphometric indices were applied to a subset of KURDS and used in conjunction with multinomial logistic regression and decision tree analyses of qualitative ped data to explore endogenous and exogenous controls on the development of soil structure. We found that the exogenous factor, climate, exhibited the greatest control over ped shape and size whereas clay content (endogenous) was the most important factor predicting ped grade. The finding that climate exhibits control over the evolution of soil structure represents an unexplored avenue for understanding how global climate change will affect morphological properties that control soil hydrology. Overall, this dissertation demonstrates the possibilities of describing peds in terms of quantitative variables and analyzing continental-scale databases of soil structure.