Distribution and Genesis of Ortstein and Placic Horizons in Soils of the USA: A Review
Soils with ortstein cover 2.2 million ha in the USA, 87% of which occur in Michigan and Florida. Of the 650 soils in the National Soil Survey database classified as Spodosols, 47 contain sufficient ortstein to be classified in the ortstein rupture-resistance class; another 42 soils contain materials that are <50% cemented and, therefore, not officially recognized as ortstein. Well-developed ortstein averages 41 cm in thickness and is moderately cemented, dense (bulk density 1.60 g cm−3), massive, very firm to extremely firm, predominantly in sandy particle-size classes, and may contain living roots. Because soils with and without ortstein often occur on the same landforms, soils containing >5% ortstein (89 series) were compared with geographically associated and competing soil series (59) that lack ortstein, using analysis of variance. Soils with ortstein occur on lesser slopes (p = 0.001) and at lower elevations (p = 0.015) than soils without ortstein. The lower depth boundary and thickness of the spodic horizon were significantly greater (p = 0.001) in soils with ortstein than in those without ortstein. The data suggest that soil water transporting cementing materials (Fe, Al, Si, and dissolved organic C) moves more slowly in landscape positions where ortstein eventually forms than in those where it is absent. However, ortstein is not restricted to soils with poor drainage as only 39% of the soils with ortstein have an aquic soil moisture regime. From a weight-of-evidence assessment, ortstein is cemented by Al-organic complexes and short-range-order compounds, and placic horizons are cemented by Fe as ferrihydrite or as Fe-organic complexes.Please view the pdf by using the Full Text (PDF) link under 'View' to the left.
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