Proposed Amendment to Soil Depth Classesin Taxonomy and Root Restricting Depth in Taxonomy and the Soil Survey Manual
Keys to Soil Taxonomy (page 310) with proposed amendment in RED:
Soil Depth Classes
Soil depth classes are used in all families of mineral soils and Histels that have a root-limiting layer at a specified depth from the mineral soil surface, except for those families in Lithic subgroups (defined below) and those with a fragipan. The root-limiting layers included in soil depth classes are duripans; petrocalcic, petrogypsic, and placic horizons; and a nearly continuous, horizontally oriented, human-manufactured layer;continuous ortstein (90 percent or more); densic, lithic, paralithic, and petroferric contacts.Soil depth classes for Histosols are given later in this chapter. One soil depth class name, “shallow,” is used to characterize certain soil families that have one of the depths indicated in the following key.
Key to Soil Depth Classes for Mineral Soils and Histels
A. Oxisols that are less than 100 cm deep (from the mineral soil surface) to a root-limiting layer and are not in a Lithic subgroup.
Shallow
or
B. Other mineral soils and Folistels that are less than 50 cm deep (from the mineral soil surface) to a root-limiting layer and are not in a Lithic subgroup.
Shallow
or
C. Other Histels that are less than 50 cm deep to a root-limiting layer.
Shallow
or
D. All other Histels and mineral soils: No soil depth class used.
Keys to Soil Taxonomy (page 300) with proposed amendment in RED:
Root-Limiting Layers
The concept of root-limiting layers as used in this taxonomy defines the base of the soil horizons considered for most (but not all) differentiae at the family level. The properties of soil materials above the base and within the control section are used for assignment of classes, such as particle-size classes and their substitutes. One notable exception to the concept of root-limiting layers is in assignment of soil depth classes (defined below) to soils with fragipans. Unless otherwise indicated, the following are considered root-limiting layers in this chapter: a duripan; a fragipan; petrocalcic, petrogypsic, and placic horizons; a nearly continuous, horizontally oriented, human-manufactured layer; continuous ortstein; and densic, lithic, paralithic, andpetroferric contacts.
Soil Survey Manual (page 134-135) with proposed amendment in RED:
The root restricting depth is where root penetration would be strongly inhibited because of physical (including soil temperature) and/or chemical characteristics. Restriction means the incapability to support more than a few fine or very fine roots if depth from the soil surface and water state, other than the occurrence of frozen water, are not limiting. For cotton or soybeans and possibly other crops with less abundant roots than the grasses, the very few class is used instead of the few class. The restriction may be below where plant roots normally occur because of limitations in water state, temperatures, or depth from the surface. The evaluation should be for the specific plants that are important to the use of the soil. These plants should be indicated. The root-restriction depth may differ depending on the plant considered.
Root-depth observations preferably should be used to make the generalization. If these are not available—and often they are not because roots do not extend to the depth of concern—then inferences may be made from morphology. Some guidelines follow for physical restriction. Chemical restrictions, such as high extractable aluminum and/or low extractable calcium, are not considered here. These are generally not determinable by field examination alone.
Physical root restriction is assumed at contact to rock, whether hard or soft. Further, certain pedogenic horizons, such as fragipans, infer root restriction. A change in particle size distribution alone, as for example loamy sand over gravel, is not always a basis for physical root restriction.
Physical root restriction may also be from nearly continuous, horizontally oriented, human-manufactured materials. Examples of these types of material are geotextile liners, asphalt, concrete, rubber and plastic.
A common indication of physical root restriction is a combination of structure and consistence which together suggest that the resistance of the soil fabric to root entry is high and that vertical cracks and planes of weakness for root entry are absent or widely spaced. Root restriction is inferred for a continuously cemented zone of any thickness; or a zone >10-cm thick that when very moist or wet is massive, platy, or has weak structure of any type for a vertical repeat distance of >10 cm and while very moist or wet is very firm (firm, if sandy), extremely firm, or has a large penetration resistance.
Purpose:
To clearly define the M layer as a root-limiting layer in the Keys to Soil Taxonomy and different types of physical root restrictions in the Soil Survey Manual for classification purposes.
References:
United States Department of Agriculture, Natural Resources Conservation Service. 2010. Keys to Soil Taxonomy. Eleventh edition, 2010.
United States Department of Agriculture, Soil Conservation Service. 1993. Soil Survey Manual. U.S. Department of. Agriculture. Handbook 18.