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Causes of Soil Boundaries in an Arid Region: II. Dissection, Moisture, and Faunal Activity¹

ABSTRACT

Studies of soils in arid regions show similar boundaries between many soils. This paper illustrates some of the boundaries caused by differences in landscape dissection, soil moisture, and faunal activity. Some of the boundaries are apparent at the land surface and others are not.

In dissected terrains a change from Argids to Orthids has been caused by slow, long-continued soil truncation associated with the dissection. The boundary occurs as a gradual, lateral change from a noncalcareous, reddish-brown, and red argillic horizon to a calcareous, brown cambic horizon.

Differences in moisture movement in the past have caused soil differences resulting in boundaries between Typic Haplargids and Petrocalcic Paleargids. During Pleistocene pluvials, soil moisture moved deeply into reddish-brown pipes (preventing carbonate accumulation) but not in adjacent petrocalcic horizons. Typic Haplargids occur in the pipes whereas Petrocalcic Paleargids occur adjacent to the pipes.

Soil boundaries have also been caused by differences in moisture movement at present. Runoff from fan-piedmonts increases vegetation and organic carbon in and along margins of basin floors. This and associated increases in clay commonly result in a boundary between Typic Haplargids on the fanpiedmont and Ustollic Haplargids on the basin floors and adjacent toeslopes.

In places, soil fauna have obliterated argillic horizons by mixing A and B horizons. This causes a boundary between Argids and Orthids.

¹ Contribution of Soil Survey Investigations, Soil Conservation Service, USDA. Presented before Div. S-5, Soil Science Society of America, 12 Nov. 1973. Acknowledgment 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.