Digital soil mapping of a red clay subsoil covered by loess D.M. Evans, Alfred E. Hartemink ⁎ University of Wisconsin-Madison, Department of Soil Science, FD Hole Soils Lab., 1525 Observatory Drive, Madison, WI 53706, USA abstract article info Article history: Received 12 December 2013 Received in revised form 13 March 2014 Accepted 15 March 2014 Available online 13 April 2014 Keywords: Alfisols Terra Rossa Catena Regression models Regression kriging Probability 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 7000 ha 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 10 cm thinner and contains 15% less clay. At 60% probability, the model predicts 2600 ha with red clay compared to 3300 ha from the Soil Survey Geographic database (SSURGO), 2000 ha with a 30 to 55 cm loess cover (vs. 1600 ha from SSURGO) and 1700 ha with greater 15 cm red clay thickness (vs. 3300 ha 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. Published by Elsevier B.V. 1. Introduction Digital soil mapping techniques have been used to map soil horizons and its soil properties (McBratney et al., 2003). One of the earlier studies was by Moore et al. (1993), who used terrain attributes to map the A horizon depth on hillslopes. King et al. (1999) used logistic regression and a set of covariates to map the presence of a clay loam horizon. Vanwalleghem et al. (2010) studied the relationship of topography and the depths of soil horizons using multiple regression. They found poor correlation between terrain attributes derived from a DEM and the soil horizon depths. Gastaldi et al. (2012) mapped the presence and the thickness of horizons using both logistic regression and linear regression in order to map entire soil profiles. The approach allows for the mapping of the presence of a horizon and its properties. This paper builds on these ideas and methods, and applies them to map a red clay subsoil in Wisconsin, USA. The Driftless Area of Wisconsin is a region of stream-dissected up- lands in the southwest part of the state with a size of about 41,000 km 2 (USGS, 2013). It has not been glaciated within the Quaternary period (Knox, 1982). The Driftless Area extends into the states of Minnesota, Iowa, and Illinois, but the greatest extent is in Wisconsin. Loess covers much of the landscape with the greatest depths in the western part of the state closest to the Mississippi River. The Driftless Area was affected by a periglacial climate and associ- ated erosional processes during the last ice age that ended about 12,000 years ago. The periglacial conditions caused some mass wasting and hillslope erosion due to solifluction (Mason and Knox, 1997). These processes reduced the thickness of loess on side slopes and re- sulted in accumulation in the lower parts of the landscape. Much of the colluvial material was removed after the glaciers receded (about 10,000 years ago) by alluvial activity (Knox, 1989). A red subsoil clay occurs on the uplands in the Driftless Area of southwest Wisconsin (Knox et al., 1990). The red clay ranges from several centimeters to a maximum of several meters thick. It is present on uplands underlain by dolostone (CaMg(CO 3 ) 2 ) and is not docu- mented to occur over sandstone. The red clay is buried by loess and it is sometimes described as a geologic unit named the Rountree Formation (Knox et al., 1990). The material has many characteristics of Terra Rossa, soils formed in a Mediterranean climate in the residuum of hard carbonate bedrock with additions of other materials (Merino and Banerjee, 2008). The red clay in the Driftless region contains weathering products from the dolostone bedrock and from the loess (Stiles and Stensvold, 2008). The high clay content (35–75%) impacts the rate of groundwater re- charge and the high iron content of the red clay makes it a potential sink for phosphorus (Frolking, 1978). More information is needed regarding Geoderma 230–231 (2014) 296–304 ⁎ Corresponding author. E-mail address: hartemink@wisc.edu (A.E. Hartemink). http://dx.doi.org/10.1016/j.geoderma.2014.03.013 0016-7061/Published by Elsevier B.V. Contents lists available at ScienceDirect Geoderma journal homepage: www.elsevier.com/locate/geoderma