How deep is your soil? Quantifying and spatially analyzing understudied deep soil in the United States

Deep soil is largely understudied despite its importance for understanding terrestrial biogeochemical processes. Here, understudied soil is defined as the difference between soil studied to a reported depth and the estimated depth to bedrock. To assess deep soil across the US, we quantified and spatially analyzed understudied soil using soil survey data and model estimates of bedrock depth. We derived an equation to estimate understudied soil using the dataset parameters “max lower depth studied”, “depth to bedrock”, and “likelihood of bedrock in the top 200 cm”. Survey data and bedrock model outputs revealed that soil has been studied to an average depth of 1.4 m, and the average depth to bedrock is 26 m in the US. The greatest amounts of understudied soil occur in the Midwest and the Southwest. Soil data density was highest in regions with greater population density, specifically, the Pacific region (excluding Alaska), the Midwest, and the Northeast. In contrast, Alaska, the Mountain region and South were underrepresented. To understand soil diversity and any taxonomic bias of the global soil data available, soil orders in the dataset were compared with US-based National Resource Conservation Service areal percentages. Oxisols, Alfisols, Ultisols, Andisols, and Histosols were overrepresented, whereas Gelisols, Aridisols, Vertisols, Entisols, and Spodosols were markedly understudied.