Distribution and Origin of Argillic Horizons across Iowa-A Novel Hypothesis

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 Alfisols (Udalfs and Aqualfs). The remaining 41\% is in Argiudolls, Argiaquolls, and Argialbolls. At the county level, the prevalence of argillic horizons varies 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 CaCO3 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 spatial interpretations. Our findings 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, calcareous conditions in closed catenas preclude clay translocation, and promote formation of calcium silicate which is a passive sink for Si.