Limiting LayerSection: Previous  -  Next

Definition

In soil evaluation, four kinds of limiting layers are recognized: bedrock, dense till, fragipan, and coarse sand and gravel. Three depth zones to a limiting layer are also recognized: 1) 0 to 20 inches, 2) 21 to 40 inches, and 3) more than 40 inches, or no limiting layer.

Importance of limiting layer

Depth to the limiting layer is important because it determines the amount of soil material favorable for plant rooting. A shallow soil limits the amount of water the soil can supply to plants.

Requirements of limiting layers
  1. Three classes of soil depth to a limiting layer are recognized:
    1. 0 to 20 inches
    2. 21 to 40 inches
    3. More than 40 inches (or no limiting layer)
  2. The limiting layer must be more than 10 inches thick. If the layer or material meets the requirements listed below and extends to the bottom of the pit, assume that it is more than 10 inches thick, thus a limiting layer.
Types of limiting layers

Bedrock
More than 80% of the layer is rock material so hard that:
  1. It cannot be cut with a spade or dug into with a knife, and/or
  2. Roots cannot grow into it.
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Dense till
Meets all the requirements of till parent material. In addition:
  1. It has bulk density of 1.75 g/cm3 or more (it feels heavy compared with other soil material).
  2. It has little pore space and few or no roots.
  3. If clay films are present, they are more than 4 inches apart.
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Fragipan
Has these characteristics:
  1. Prisms that, on average, are 4 inches or more wide.
  2. Material in the prisms is brittle.
  3. There are few or no roots in the prisms.
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Coarse sand and gravel

Has these characteristics:
  1. Qualifies for the sandy texture group and the sands are mainly greater than 0.5 mm in diameter (roughly equivalent to 40-grit sandpaper).
  2. Gravel is usually present but may be lacking.
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