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Causes of Soil Boundaries in an Arid Region: I. Age and Parent Materials¹

ABSTRACT

Studies of soil distribution in arid regions show recurring similarities of many soil boundaries. This paper illustrates some of the boundaries caused by differences in soil age and parent materials. The relations are shown by small-scale diagrams of landscapes and by large-scale diagrams of the soil boundaries, diagnostic horizons, and other morphological features.

A major cause of difference in age is the deposition of new parent materials in some places but not in others. As a result, soils of the younger deposits are adjacent to unburied soils that are more complex. Particularly prominent changes in soils occur across the boundary between soils of Holocene and Pleistocene age, with taxonomic differences ranging from soil family to order. Magnitude of the morphological change between soils of these two ages and occurrence of the boundary over vast areas of the Southwest indicate the pedogenic significance of the boundary.

Other boundaries are caused by changes in parent materials. Differences in content of coarse fragments can cause category changes ranging from soil family to great group. High-carbonate parent materials can cause taxonomic differences on the level of suborder or order. Some soil boundaries have no expression at the land surface, but others are marked by landforms such as terraces, scarps, and fans.

¹ Contribution of Soil Survey Investigations, Soil Conservation Service, USDA. Presented before Div. S-5, Soil Science Society of America, 12 Nov. 1973. Acknowledgement is made to Sam Horn for drawing landscapes in the block diagrams.

² Soil Scientist, Soil Survey Investigations, SCS, and Adjunct Professor of Agron., Texas Tech Univ., Lubbock 79409.

Received for publication April 15, 1974. Accepted for publication October 24, 1974.