Soils 205- General Soils

Lecture 23- The Soil Orders

To view more pictures and review the properties of each soil order please use the following website:

http://soils.ag.uidaho.edu/soilorders/index.htm

I.  The soil orders

    a.  Gelisols

        -soils with permafrost within 2 m of the surface

        -typically have Bf or Cf horizons

        -occupy ~8.6% of land area, 7.5% in US

    b.  Histosol

        -organic soils, such as peats, mucks

        -consist of O horizons at least 40 cm thick

        -occupy ~1.2% of land area, 7.5% of US

    c.  Spodosol

        -soils with an E horizon and illuvial sesquioxides

        -usually very sandy

        -acid forest litter

        -occupy ~2.5% of land area, 3.3% in the US

    d.  Andisol

    -soils formed in volcanic ash

    -distribution limited to areas with volcanic activity

    -dominated by poorly crystalline minerals

    -occupy 0.7% of land area, 1.7% in US

    e.  Oxisol

        -INTENSELY weathered soils of tropical areas

        -typically have A-Bo horizon combination

        -occupy ~7.6% of land area, <0.01% in US

    f.  Vertisol

        -clayey soils with high shrink/swell

        -churned up, generally lack distinct horizonation

        -present many engineering difficulties

        -occupy ~2.4% of land area, 1.7% in US

    g.  Aridisols

    -soils of arid climates with moderate to strong development

    -have accumulations of CaCO₃ (Bk horizons), sometimes salts (Bz), cementation (Bkqm or Bkm), and clay (Bt)

    -occupy ~8.5% of land area, 8.8% of US

    h.  Ultisols

    -forest soils with a Bt horizon and relatively low Ca and Mg

    -often found in higher rainfall areas

    -typically have O-A-E-Bt horizons

    -occupy ~8.5% of land area, 9.6% of US

    i. Mollisols

    -soils of grassland ecosystems

    -have thick, dark A horizons

    -usually have high native fertility

    -occupy ~6.9% of land area, 22.4% of US

    j.  Alfisols

        -forest soils with a Bt horizon and relatively high Ca and Mg

        -typically have O-A-E-Bt horizons

        -occupy ~9.7% of land area, 14.5% in US

    k.  Inceptisol

        -soils with minimal B horizon development

        -usually A-Bw-C

        -often in mountainous areas

        -occupy ~9.9% of land area, 9.1% of US

    L.  Entisol

        -soils with minimal development

        -usually have an A over C or R

        -found on floodplains and steep slopes

        -16% of ice-free land surface, 12% of US