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ABSTRACT
Although pedogenesis and landscape evolution have occurred on the eastern North Dakota till plain since the inception of the Holocene, soils with argillic horizon arc recognized on a very limited basis. Argillic soils are not mapped in well-drained positions on the youngest drift prairie landscapes. Roughly half of 47 pedons examined at a precision farming research site in Barnes County, North Dakota show evidence of genetic argillic horizons based on soil structure, hand texturing, and cutan presence. This study applies physical and micromorphological evidence to verify that diagnostic argillic horizons have formed in well-drained settings on this eastern North Dakota landscape. Total clay distributions and fine/total clay ratios show marked increases in genetic argillic horizons compared with surface horizons and parent materials. The argillic horizons average about 28 cm in thickness; only two pedons were thinner than 7.5 cm. Consequently, the increase in total clay and fine clay distributions, and the thickness criteria required to meet diagnostic horizon status as established by Soil Taxonomy are satisfied. The upper argillic horizon boundary of seven profiles was 50 cm or more below the surface, possibly indicating relict argillic horizons. Micromorphologic evidence shows the presence of illuvial clay in the three soils sampled for microscopy. Porosity variations caused by the argillic horizons likely affect subsurface flow and may be responsible for variable N levels measured for these soils. These soils formed in well-drained landscape settings, where stratified drift may predispose the soils to illuviation.
GUY SMITH (1986) stated that argillic horizons were not "weighted more heavily" in Soil Taxonomy than other diagnostic subsurface horizons. However, the definition, recognition, and genesis of horizons featuring accumulated silicate clays have occasionally dominated pedological discourse. Smith (1986) believed the argillic horizon was important not so much for what it was, but for what it does: increase nutrient status, enhance water retention, reduce permeability and "perch" water, as well as reflect geomorphic stability of the landscape. These interpretations, crucial to understanding water and solute movement at the landscape scale, explain why soil stratigraphy has taken on new meaning for environmental and hydrogeological assessments. Many recent studies have attempted to quantify the role argillic horizons, and other restrictive subsurface horizons, exert in soil water flow (McDaniel and Falen, 1994; Perillo et al.,...