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A Simple Textural Index for Assessing Chemical Weathering in Soils¹

ABSTRACT

Chemical weathering affects the particle size distribution of soils, but little use has been made of this fact other than the inferences of increased chemical weathering generally made from increase in clay content. We postulated a simple model for assessing chemical weathering from particle size distribution. In the model, we assumed that a rind tends to peel-off a mineral when the mineral is under attack by an advancing weathering front parallel to its surface. Using the model, we calculated theoretical particle size variations of some loessial soils. The calculations, as expected, showed that fine fractions (< 50 µm) are much more affected by chemical weathering than are coarse fractions (> 50 µm). We concluded from this that silt fractions may be used for assessing chemical weathering, but if they must be used for assessing uniformity of parent material, they should be used advisedly.

We applied our model to 161 loessial soils of the Mississippi Valley, all having udic or aquic moisture regimes. We used the fine-silt/total-silt ratio trend for the upper part of the solum as a weathering index. The index correlates with mean annual temperature (r = 0.70) which in humid regions is assumed to be the main factor involved in the chemical weathering of soils. Although our index is not universally applicable, it may help in comparing the weathering status of soils developed on similar parent materials.

¹ Contribution from National Soil Survey Laboratory, Midwest Technical Service Center, Soil Conservation Service, USDA, Federal Bldg., Lincoln, Nebraska 68503. The senior author was supported by a grant from the Program for Culutral Cooperation between the United States and Spain.

² Associate Professor, Escuela Tecnica Superior de Ingenieros Agronomos, Universidad de Cordoba, formerly Visiting Professor, NSSL, Lincoln, Nebraska, and Research Soil Scientist, USDA, SCS, NSSL, Lincoln, NE 68508, respectively.

Received for publication June 5, 1978. Accepted for publication September 27, 1978.

This article has been cited by other articles:

				D. R. Muhs, E. A. Bettis III, J. Been, and J. P. McGeehin Impact of Climate and Parent Material on Chemical Weathering in Loess-derived Soils of the Mississippi River Valley Soil Sci. Soc. Am. J., November 1, 2001; 65(6): 1761 - 1777. [Abstract] [Full Text] [PDF]