Diagnostic Subsurface Horizons

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Argillic horizon - An argillic horizon is normally a subsurface horizon with a significantly higher percentage of phyllosilicate clay than the overlying soil material. It shows evidence of clay illuviation. The argillic horizon forms below the soil surface, but it may be exposed at the surface later by erosion.

Required Characteristics

  1. All argillic horizons must meet both of the following requirements:

    1. One of the following:

      1. If the argillic horizon is coarse-loamy, fine-loamy, coarse-silty, fine-silty, fine, or very-fine or is loamy or clayey, including skeletal counterparts, it must be at least 7.5 cm thick or at least one-tenth as thick as the sum of the thickness of all overlying horizons, whichever is greater. For example, if the eluvial and illuvial horizons together are more than 150 cm thick, then the argillic horizon must be 15 cm or more thick; or

      2. If the argillic horizon is sandy or sandy-skeletal, it must be at least 15 cm thick; or

      3. If the argillic horizon is composed entirely of lamellae, the combined thickness of the lamellae that are 0.5 cm or more thick must be 15 cm or more; and

    2. Evidence of clay illuviation in at least one of the following forms:

      1. Oriented clay bridging the sand grains; or

      2. Clay films lining pores; or

      3. Clay films on both vertical and horizontal surfaces of peds; or

      4. Thin sections with oriented clay bodies that are more than 1 percent of the section; or

      5. If the coefficient of linear extensibility is 0.04 or higher and the soil has distinct wet and dry seasons, then the ratio of fine clay to total clay in the illuvial horizon is greater by 1.2 times or more than the ratio in the eluvial horizon; and

  2. If an eluvial horizon remains and there is no lithologic discontinuity between it and the illuvial horizon and no plow layer directly above the illuvial layer, then the illuvial horizon must contain more total clay than the eluvial horizon within a vertical distance of 30 cm or less, as follows:

    1. If any part of the eluvial horizon has less than 15 percent total clay in the fine-earth fraction, the argillic horizon must contain at least 3 percent (absolute) more clay (10 percent versus 13 percent, for example); or

    2. If the eluvial horizon has 15 to 40 percent total clay in the fine-earth fraction, the argillic horizon must have at least 1.2 times more clay than the eluvial horizon; or

    3. If the eluvial horizon has 40 percent or more total clay in the fine-earth fraction, the argillic horizon must contain at least 8 percent (absolute) more clay (42 percent versus 50 percent, for example).

Keys to Soil Taxonomy, Eighth Edition