Fragipan Genesis in Polygenetic Soils on the Springfield Plateau of Missouri

Abstract

AbstractSoils on broad ridgetops of the Springfield Plateau in southwestern Missouri typically are covered by a silty mantle about 1 m thick, have developed from multiple parent materials, and contain a fragipan. The objectives of this study were to identify the parent materials, assess the influence of parent material stratigraphy on fragipan expression, and determine processes of fragipan genesis. Two pedons each of Creldon (fine, mixed, mesic Mollic Fragiudalfs) and Captina (fine‐silty, siliceous, mesic Typic Fragiudults) soils were investigated using field, soil characterization, and micromorphological techniques. The parent materials were Late Wisconsinan loess, erosional lag concentrate, and cherty limestone residuum. A composite paleosol developed in the lower two parent materials was merged with the modern soil. Since the composite paleosol was acid and highly weathered, illuviation of carbonates from the overlying loess had minimal effect on the paleosol. Clay and silt, however, were illuviated from the loess into the composite paleosol. The parent material strongly influenced fragipan expression. A fragipan formed only in the erosional lag concentrate, which was probably deposited as a slurry. Micromorphological examination showed close packing of particles in the fragipan. Depth distributions of Al, Si, Mn, and Fe extracted in sodium citrate‐bicarbonate‐dithionite were not useful in establishing if one or more of these constituents was a chemical cement of the fragipan. Close packing of skeleton grains was most responsible for fragipan genesis in these soils and could have resulted from physical ripening of the erosional lag concentrate.