Top-Down, Bottom-up, and Longer-Term Views of Geological Controls on Soils.

The geologic substrate or parent material from which a soil forms provides both the initial chemistry and the physical architecture for pedogenesis.  The identity of the directly underlying bedrock or sediment and the geomorphology of the surrounding landscape are a key starting point for interpretation of soil properties and predictions of soil response to perturbation.  Dissolved and particulate atmospheric inputs also provide parent material to soil and can play and important role in soil fertility, yet these inputs are often hidden and poorly quantified due complex controls on deposition rates and spatial and temporal heterogeneity. The influence of deep geologic processes on soil properties is also largely hidden, often beyond feasible access, but our quantitative understanding of how deep processes control on surface soil properties is improving.  Deep critical zone research highlights the importance of chemical weathering and formation of secondary minerals up to 10’s of meters below the surface.  Geophysical analyses of the deep critical zone reveal the relationships between fracture abundance and orientation and fluid flow, with feedbacks controlling both soil weathering status and topographic relief.  At the same time, we are increasingly able to analyze parent material conversion to soil at depth in more quantitative terms using geochemical approaches such as uranium-series disequilibrium.  Finally, our view of the temporal aspects of soil parent material is becoming more comprehensive, including the entire geologic history, not just the rock or sediment type. This encompasses the post-depositional or post-emplacement geologic history, which can dramatically alter trajectories of pedogenesis.  Combined, this more expansive view of geologic inputs to soil is improving our ability to understand critical functions of soils such as nutrient and water supply to ecosystems.