Boundaries of Horizons and Layers

"A boundary is a surface or transitional layer between two adjoining horizons or layers. Most boundaries are zones of transition rather than sharp lines of division. Boundaries vary in distinctness and in topography.

Distinctness.—Distinctness refers to the thickness of the zone within which the boundary can be located. The distinctness of a boundary depends partly on the degree of contrast between the adjacent layers and partly on the thickness of the transitional zone between them. Distinctness is defined in terms of thickness of the transitional zone:

Abrupt: Less than 2 cm thick

Clear: 2 to 5 cm thick

Gradual: 5 to 15 cm thick

Diffuse: More than 15 cm thick

Abrupt soil boundaries, such as those between the E and Bt horizons in many soils, are easily determined. Some boundaries are not readily seen but can be located by testing the soil above and below the boundary. Diffuse boundaries, such as those in many old soils in tropical areas, are most difficult to locate and require time-consuming comparisons of small specimens of soil from various parts of the profile until the midpoint of the transitional zone is determined. For soils that have nearly uniform properties or that change very gradually as depth increases, horizon boundaries are imposed more or less arbitrarily without clear evidence of differences.

Topography.—Topography refers to the irregularities of the surface that divides the horizons. Even though soil layers are commonly seen in vertical section, they are three-dimensional. Topography of boundaries is described with the following terms:

Smooth: The boundary is a plane with few or no irregularities.

Wavy: The boundary has undulations in which depressions are wider then they are deep.

Irregular: The boundary has pockets that are deeper than they are wide.

Broken: One or both of the horizons or layers separated by the boundary are discontinuous and the boundary is interrupted" (Soil Survey Staff: 1993)

Students of Agronomy, Soils, and Environmental Sciences (SASES) Region IV Judging

Table 3. Boundary distinctness with ranges of allowable grading tolerances. Acceptable abbreviations are in parenthesis after the distinctness class name.

Distinctness class name and Distinctness class Range for grading

Abrupt (A) < 2 cm thick ± 1 cm

Clear (C) 2-5 cm thick ± 3 cm

Gradual (G) 5-15 cm thick ± 8 cm

Diffuse (D) > 15 cm thick ± 15 cm

The boundary distinctness of the deepest horizon will not be described and should be left blank or with a dash (-) recorded in the cell.

Boundary topography will not be described for the SASES Region IV contest. Be sure to use horizon and layer depths measured at the restricted area of the pit wall since wavy and irregular boundaries may exist. Where wavy or irregular boundaries exist in restricted area the median of the maximum and minimum is the official depth.

References

Soil Survey Division Staff. 1993. Chapter 3 - Examination and Description of Soils. In Soil Survey Manual, United States Department of Agriculture Handbook No. 18 pp133-134. [Last visited January 29, 2014]