Fragipans in Pennsylvania Soils

Abstract

Fragipans are a common feature in Pennsylvania, occurring in 30% of Pennsylvania' soils. These fragipans resist root penetration and are characterized by very coarse prismatic structure, firm to very firm brittle consistence, low permeability, bulk densities that are higher than overlying horizons, loamy textures, and a low organic matter content. An index of fragipan expression (weak, moderate, and strong), based on firmness, brittleness, permeability, resistance to root penetration, and strength, is proposed. Fragipans form in transported parent materials: glacial till, colluvium, loess, old alluvium, and, less commonly, in lacustrine and turbated residual materials. Fragipan formation is favored in climates that promote a leaching environment under forest vegetation. Topographic effects on fragipan formation are suggested by the relationship between fragipan expression and depth and drainage. The degree of fragipan expression follows the trend: somewhat poorly drained > moderately well drained > well drained, while the depth to the top of the fragipan follows the opposite trend. Studies suggest that a weakly developed fragipan requires 6000 yr to form, while a strong fragipan requires 18000 yr. A four‐phase fragipan formation model is proposed for Pennsylvania fragipans. In Phase 1, transported parent material dries from the surface downward, forming prisms and the prism material is packed slightly. In Phase 2, clay and amorphous aluminosilicates are added to the prisms plugging some pores and creating brittleness by forming grain‐to‐grain contacts. Maximum fragipan expression occurs during this phase. The upper part of the prism begins to degrade during Phase 3 as more illuvial clay accumulates resulting in increased expansion and contraction during wetting and drying. The prisms are physically broken up and become a part of an overlying argillic horizon. The amorphous materials that formed the grain‐to‐grain bridges are leached from the upper part of the prism. By Phase 4, these processes have completely destroyed the fragipan.

Complex pedostratigraphy of a terrace fragipan at the Memorial Park site, central Pennsylvania

“…The current definition of a fragipan (Soil Survey Staff, 1996) is less ambig-uous for identification purposes, but still does not provide the quantitative criteria necessary to evaluate the degree of development (a morphological interpretation), and the phase of development (a time function). Ciolkosz et al (1995) defined classes of degree of expression of Pennsylvania fragipans on qualitative impressions of firmness, brittleness, apparent permeability as indicated by mottling, resistance to root penetration, and toughness or strength (resistance to digging or penetration).…”

“…The time required for fragipan formation is a topic of interest in Pennsylvania because few fragipans in the northeast have been dated (Bilzi and Ciolkosz, 1977;Ciolkosz et al, 1995). Of particular interest is the time required for incipient stages of fragipan development since, according to Ciolkosz et al (1995), data available do not allow a strong extrapolation of the rate of development with regard to either thickness or degree of development.…”

“…Of particular interest is the time required for incipient stages of fragipan development since, according to Ciolkosz et al (1995), data available do not allow a strong extrapolation of the rate of development with regard to either thickness or degree of development. Mid-Atlantic pedologists suggest that weakly to moderately developed fragipans require about 6000 years to form while strongly developed fragipans require 18,000 years (Ciolkosz et al, 1995;Foss et al, 1995).…”

“…The color of the BPF varied from grayish brown to gray bordered by strong brown. There were no Fe-Mn nodules, or mangans, which are often associated with restricted drainage (Ciolkosz et al, 1995;Lindbo and Veneman, 1989). Even though clay films were present in some profiles (Btxb horizon), there was not a sufficient increase in clay to define an argillic horizon (Soil Survey Staff, 1996).…”

Influence of redox processes on clay mineral transformation in Retisols in the Carpathian Foothills in Poland. Is a ferrolysis process present?

“…Fragipan shows low permeability which may lead to periodic water stagnation causing seasonal anaerobic conditions which favor reduction of iron oxides and their subsequent translocation and oxidation. Thus, it is very likely that ferrolysis may take place in Retisols containing a fragipan, because of the high bulk density and low porosity of the pan (e.g., Witty and Knox 1989;Lindbo et al 1994;Ciolkosz et al 1995;Szymański et al 2011). The seasonal stagnation of water within Retisols causes the bleaching of soil material indicating reduction and dissolution of iron oxides and their subsequent re-oxidation and immobilization in the form of iron nodules, concretions, soft masses, and pore linings (Cescas et al 1970;Schwertmann and Fanning 1976;Rhoton et al 1993;Zhang and Karathanasis 1997;Lindbo et al 2000;Dixon and White 2002;Szymański et al 2011Szymański et al , 2012Szymański et al , 2014Szymański and Skiba 2013).…”