Soil Science Society of America Journal Soil Sci. Soc. Am. J. doi:10.2136/sssaj2012.0044 Received 8 Feb. 2012. *Corresponding author (amostafa@zu.edu.eg). © Soil Science Society of America, 5585 Guilford Rd., Madison WI 53711 USA All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Permission for printing and for reprinting the material contained herein has been obtained by the publisher. Distribution and Origin of Argillic Horizons across Iowa—A Novel Hypothesis Pedology T he argillic horizon is a subsurface diagnostic soil horizon with clay accu- mulation of deined magnitude (Soil Survey Staf, 1999). Argillic hori- zons are extensive worldwide, being found in humid through arid regions (Allen and Fanning, 1983; Gile and Grossman, 1968). Argillic horizons likely de- velop through one or more of the following three processes: clay translocation, clay transformation, or clay neoformation (Smeck et al., 1968; Nettleton et al., 1975; Egli et al., 2001). Clay translocation, also known as lessivage, is generally described as beginning with chemical or physical dispersion of ine clays along a macropore, followed by a downward movement as a suspended load in leaching pore water, and ending with deposition (e.g., see, Eswaran and Sys, 1979; Miller, 1983; Phillips, 2007). he depth of clay deposition in the solum is thought to be controlled by wherever pore size becomes so small as to constrict water low– for example, a pore is partially plugged by illuviated clays from prior episodes of translocation– or, pore water is pulled into side pores via capillarity, which leaves the clay’s lining the original macropore ( horp and Smith, 1949). hese ideas are consistent with Rousseau et al. (2004), who, in a laboratory experiment using an intact soil column and repeated water additions from a rainfall simulator, found translocation of clay depends on rainfall intensity, ambient moisture content, occurrence of continuous Mostafa A. Ibrahim* Soil and Water Science Dep. College of Agriculture Zagazig Univ. Zagazig, Egypt 44511 C. Lee Burras 1126 Agronomy Hall Iowa State Univ. Ames, IA 50011 Argillic horizons are common in most of the 48 contiguous states of the United States. In Iowa, soils with argillic horizons cover 33.5% of the land area. Fifty-nine percent of the area in Iowa with argillic horizons is in Alisols (Udalfs and Aqualfs). The remaining 41% is in Argiudolls, Argiaquolls, and Argialbolls. At the county level, the prevalence of argillic horizons var- ies widely. We hypothesize that the presence of argillic horizons in Iowa is largely a function of these controls in some closed depressions (prairie pot- holes): (i) Si consumption by hydrophytic organisms, and (ii) prevalence of CaCO 3 and secondary Ca minerals. All controls are predicated on catena drainage. Our methodology was to compile pedologically relevant data and pedon descriptions from a variety of databases and use GIS to facilitate spa- tial interpretations. Our indings agree with the concept that mature forest and prairie catenas in open drainage systems generally have soils with well- expressed argillic horizons across an entire soil catena. In contrast, soils in catenas in closed drainage systems very rarely have argillic horizons and where they do, they are Argialbolls. The biotic (i.e., active pedogenic factor) contrast between the open and closed catenas appears to be the prevalence of Si hyper accumulators such as reeds (Sparganium eurycarpum), sedges (Carex stricta), cattails (Typha latifolia), diatoms, and sponges. Also, calcar- eous conditions in closed catenas preclude clay translocation, and promote formation of calcium silicate which is a passive sink for Si. Abbreviations: DML, Des Moines Lobe; SIDP, Southern Iowa Drift Plain.