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This article in SSSAJ

  1. Vol. 60 No. 1, p. 206-218
    Received: Aug 23, 1993

    * Corresponding author(s): maw@nssc600.nssc.nrcs.usda.gov
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Soil Properties and Genesis of Pans in Till-Derived Andisols, Olympic Peninsula, Washington

  1. M. A. Wilson ,
  2. R. Burt,
  3. R. J. Engel,
  4. T. M. Sobecki and
  5. K. Hipple
  1. USDA Natural Resources Conservation Service (NRCS), National Soil Survey Center, Federal Building, Rm. 152, MS 41, 100 Centennial Mall N, Lincoln, NE 68508-3866
    USDA-NRCS, West National Technical Center, Portland, OR
    USDA-NRCS, Spokane, WA



Subsoil pans regarded as cemented are present in certain Andisols of the Olympic Peninsula region of western Washington. Soils formed in basalt-derived alluvium or colluvium over Pleistocene-aged compact alpine till. Theories of pan formation include pedogenic cementation by silica, allophane, or humic-metal complexes; or geogenic till compaction with no subsequent cementation. We examined the properties of three pedons with compacted and cemented subsoil horizons to better understand the genesis of these soils and pan horizons. Pan horizons, designated as Cm, are very brittle, nearly impossible to dig with hand tools, and massive in structure. Soils are acidic, with pH(H2O) for all horizons ranging from 3.7 to 6.0. Crystalline phyllosilicates identified are gibbsite, kaolinite, chlorite, vermiculite, and hydroxy-interlayered vermiculite. Allophane was detected in most mineral horizons, including Cm horizons, and allophane Al/Si molar ratios vary from 1.2 to 3.4. Low NaOH-extractable Si, low Si/Al molar ratios from NaOH extracts, high rainfall, and the presence of gibbsite decrease the likelihood of pedogenic opal as a cementing agent in pans. Limited organo-metallic deposition, low organic C, and low pyrophosphate-extractable Fe and Al in pan horizons suggest that podzolization processes related to ortstein formation are not responsible for Cm horizon cementation. Micromorphologic examination of Cm horizons detected a light brown to nearly colorless substance that appears to be allophane around mineral grains and in pore channels. Soil chemistry data support allophane as the primary cementing agent in these pan horizons, with kaolinite, gibbsite, and Fe oxides acting as possible accessory agents. Glacial compaction, parent material mineralogy, and cycles of wetting and drying are important factors affecting cementation of these pans.

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