PODZOLIZATION IN ULTRAMAFIC MATERIALS IN SOUTHEAST ALASKA

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Abstract

Ultramafic materials weather rapidly to maintain relatively high saturation with exchangeable Mg, and they have very high Fe and high Mn contents. These features retard the translocation of humus compounds, which is essential for podzolization. Three soils in a range of ultramafic materials were sampled to relate their morphology and spodic properties to parent material differences. Pedon 1 in residuum from weathered dunite is a coarseloamy, oxidic Dystric Cryochrept with no E horizon. Pedon 2 in ultramafic till with a cemented substratum is a loamy, serpentinitic, frigid, shallow Aquic Durochrept. It has E and Bs horizons, but the pH in the Bs horizon is too high (pH > 6) for spodic materials. Pedon 3 in mixed (partially ultramafic) till with compact substratum is a loamy, mixed, shallow Typic Cryaquod. It has an E and a spodic B horizon. The B horizons of all three pedons have low acid oxalate extractable Al (Alo) and very high iron (Feo) contents. They have low pyrophosphate-extractable to dithionite-extractable Fe ratios (Fep/Fed < 0.5) as a result of very high Fed contents. The highest Feo and Fed in each pedon are 92 and 284 g/kg in the Bw horizon of Pedon 1, 98 and 84 g/kg in the Bs horizon of Pedon 2, and 34 and 38 g/kg in the Bhs horizon of Pedon 3. Feo/Fed ratios > 1, as in the Bs horizon of Pedon 2 and in the C horizons of all three pedons, are due primarily to high magnetite contents in the soils. Weathering and leaching in the perhumid climate of southeast Alaska have not been sufficient to form a Podzol or a Spodosol in the soil on dunite. The free Mn and exchangeable Mg contents are lowered sufficiently to form a Podzol in the ultramafic till, but the pH remains too high for a Spodosol. A soil with Podzol morphology and Spodosol chemistry has developed in the mixed till.

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