Digital soil mapping of a red clay subsoil covered by loess

Abstract

A red clay subsoil is commonly found in the soils of the Driftless Area of Wisconsin, USA. The red clay is buried by a layer of loess and occurs on uplands underlain by dolostone. It is assumed to have derived from the impurities of the weathered dolostone. The red clay has implications for groundwater recharge and lateral flow of water through the landscape as well as the fixation of excess phosphorus from animal manure and inorganic fertilizers. Here we have produced digital soil maps at different levels of probability of the red clay presence, the depth to the red clay, and its thickness. Five hundred and sixty-one soil observations were made in a 7000ha study area and were used to investigate the relationship of the red clay to landscape characteristics and short range soil variability. Soil observations and environmental data were used to create digital soil maps using regression models and kriging. Compared to the described soil series for the study area it was found that the red clay is less red, about 10cm thinner and contains 15% less clay. At 60% probability, the model predicts 2600ha with red clay compared to 3300ha from the Soil Survey Geographic database (SSURGO), 2000ha with a 30 to 55cm loess cover (vs. 1600ha from SSURGO) and 1700ha with greater 15cm red clay thickness (vs. 3300ha from SSURGO). It is concluded that SSURGO overestimates the total area with red clay and the thickness of the clay. This study also showed how a clay subsoil can be mapped at different levels of probability.