Monday, November 13, 2006

Deep Regolith Pedology, Paleopedology, and Modern Soil Survey.

S.J. Indorante1, D.R. Williams1, and D.A. Wysocki2. (1) USDA-NRCS, Carbondale MLRA Office, 148 E. Pleasant Hill Road, Suite 105, Carbondale, IL 62903, (2) USDA-NRCS, National Soil Survey Center, 4631 S 50th Street, Lincoln, NE 68516

Most soil descriptions in field soil survey terminate at 2 meters or less, which is adequate for classification and interpretation. Detailed observations and descriptions to greater depths are needed to understand soil to substrata relationships and the whole soil-landscape system.  We undertook a deep loess sequence investigation that demonstrates the importance of observing, describing, and storing key soil descriptive data and then linking it to accompanying physical and chemical data.  The study area was a 12 meter thick geologic section near the Ohio and Mississippi River confluence in southern, Illinois.  The dominant loess source is the Mississippi River Valley.  The stratigraphic section from top to bottom includes Peoria Loess/Roxanna Silt/Loveland loess/Mounds Gravel.  National Cooperative Soil Survey guidelines were used for soil description and soil sampling.  The description was entered into the Pedon Description Program of the National Soil Information System (NASIS).  Complete description reports were generated from NASIS and then the descriptions and physical and chemical data were correlated with stratigraphic observations.  Detailed soil description of the entire section revealed various degrees of pedogenic modification both in paleosols and loess itself. The Alford soil was identified as the modern soil and two unnamed paleosols occur under it in the Peoria Loess.  A Farmdale paleosol occurs at the Peoria Loess and Roxanna silt contact and the Sangamon geosol occurs at the contact of the Roxanna silt and Loveland loess. The Sangamon geosol extends through the Loveland loess in to the underlying Mounds Gravel. Detailed description of soils (top 2 meters) and the underlying materials including deeper soil horizons are needed to understand the soil-landscape system as a whole.  Detailed descriptions of the deep regolith revealed the depths to which pedological processes operate, and in particular those features and morphology that indicate or control water movement.