Factors Affecting the Formation of Dark, Thick Epipedons

Beneath Forest Vegetation, Michigan, USA

R. J. Schaetzl

Extremely gravelly, coarse-textured soils (frigid Udorthents and Rendolls) with different thicknesses of Oa + A horizon sequences were studied to identify factors that have influenced their genesis. These well-drained, forested soils occur on geomorphic surfaces that range in age from 3200 to 6000 years BP. These soils all have more than 500 g kg ^-1 coarse fragments by mass; most contain less than 300 g kg ^-1 fine earth. In the lower solum of most pedons, content of cobbles increases and amount of fine earth decreases. Most coarse fragments are dolomite and chert.

Thick, gravelly Oa and A horizons are weakly correlated with parent material characteristics such as high pH and carbonate contents. Organic matter concentrations in, and thicknesses of, upper horizons are enhanced by an abundance of coarse clasts, as soils with the most gravel exhibited the thickest and darkest epipedons. Relatively high amounts of crystalline clasts in the fine gravel fine gravel fraction, as well as feldspathic minerals in the fine sands, also appear to promote the development of mollic epipedons.