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DIVISION S-5—PEDOLOGY

Genesis of Bisequal Soils on Acidic Drift in the Upper Great Lakes Region, USA

Dep. of Soil Sci., 1525 Observatory Dr., Univ. of Wisconsin, Madison, WI 53706-1299

^* Corresponding author (bockheim{at}facstaff.wisc.edu)

Seventeen bisequal pedons were characterized on drift primarily from the Greatlakean advance [ca. 12 000 yr before present (BP)] in northern Wisconsin and the upper Michigan. The pedons, Alfic Haplorthods and Alfic Oxyaquic Fragiorthods, contain an Orthod sequum over an Udalf sequum and are derived primarily from coarse-loamy or coarse-silty materials overlying stratified sand and gravel or coarse-loamy materials. The upper sequum contains abundant roots and is very strongly acidic (pH 4.8), low in clay (<4.8%) and base cations (<20% base saturation), and high in exchangeable acidity (13 cmol_c kg^-1) and exchangeable Al (60% saturation). The lower sequum contains few roots, is strongly acidic (pH 5.4), is more enriched in clay (8.0%) and base cations (45% base saturation), and has lower exchangeable acidity and Al than the upper sequum. The occurrence and position of argillic and fragic horizons in the profile are favored by a lithologic discontinuity. The two horizons may form exclusive of one another. On the basis of soil solution chemistry data and mass-balance studies, the argillic horizon has formed from neoformation of clays with some clay migration. The fragic materials are of pedogenic origin and are forming in the upper part of a degrading argillic horizon. The soils have undergone environmental changes during the past 12 000 yr, but they appear to have originated from processes that are occurring today that result from extensive leaching during melting of the spring snowpack and solute movement in two-storied parent materials. The soils are strongly developed in view of the short snow-free season.

Abbreviations: BP, before present • CEC, cation exchange capacity

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