Exogenous and endogenous controls on the development of soil structure

The categorical and qualitative nature of currently available soil structural data along with the lack of a geographically broad dataset have impeded progress in understanding the development of soil structure. In this study, we assembled a soil, climate, and ecological dataset for the USA, and used it to analyze relationships between soil structure (ped type, shape, size, and grade) and exogenous and endogenous variables influencing the development of soil structure. We analyzed a subset of the National Cooperative Soil Survey (NCSS) Soil Characterization database after merging this information with climatological and ecological data. The merged and cleaned dataset contains > 4400 observations from approximately 1600 pedons. We found that climate, as an exogenous factor was the most important predictor of ped shape and size. Cold and/or dry climates promoted the development of larger anisotropic peds with rougher surfaces whereas warmer and more humid climates promoted the development of finer equidimensional peds with smoother surfaces. Based on these findings, we argue that climate promotes the development of soil structure along either fragmentation or aggregation pathways. The former pathway is characterized by largely mechanical processes in cold and dry environments, whereas aggregation is promoted by predominately biological and chemical mechanisms found in warmer and wet environments. This connection between climate and the development of soil structure represents a potentially important effect of climate on a morphological property strongly linked to soil hydrology that warrants further investigation with continental-scale soil data.