The Lower Boundary of Selected Mollisols

Abstract

Abstract Thicknesses of three Mollisols (Tama, Elburn, and Drummer series), developed from Wisconsinan‐age material in a toposequence in Central Illinois, were considerably greater (18 to 68 cm) when their lower boundaries were determined by the depth of rooting of native perennial big bluestem ( Andropogon gerardi ) than when determined by the lower limit of the solum. A combination of four criteria—structural development, significant clay accumulation, significant clay films, and the presence of completely unleached material—probably gave the best measure of solum thickness of these soils. However, evidence of some clay movement below the solums and the greater depth of rooting of native perennial grass, and also of such crops as corn ( zea mays L.), suggests that material beneath the solum is important in the behavior, definition, and classification of many of these kinds of soils in the north‐central region of the U.S.