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Laboratory Studies and Genesis of a Clay-Enriched Horizon in the Lowest Part of the Solum of Some Brunizem and Gray-Brown Podzolic Soils in Illinois¹

ABSTRACT

Results are presented from laboratory investigations of Beta horizons (clay-enriched layers found below the B₂ horizons) of selected soils in the Wisconsin-age glaciated region of northern Illinois.

Stereoscopic microscopic investigations indicated the peds were microconstructional. This was interpreted to be the result of not only the arrangement of skeletal material in situ, but also the effect of colloidal material which has moved into the horizon from above and deposited on surfaces of grains, peds, and walls of pores to alter the microconfiguration and porosity. Microscopic studies of thin sections showed a rather porous fabric with a large percentage of the clay being strongly oriented in layers along surfaces of mineral grains, walls of pores and root channels, and surfaces of peds. Thin sections of sand columns through which clay suspensions had been passed had a fabric markedly similar to that of Beta horizons. These micromorphological investigations strongly suggest that the Beta horizon is primarily a zone of clay illuviation.

Mechanical analyses indicated that the total clay fraction (< 0.002 mm.) was at a maximum in the B₂ horizon and that fine clay was at a maximum in the Beta horizon of each profile. Amounts of organic matter by weight were about the same in the B₂ and Beta horizons. On the other hand, if amounts of clay and organic matter were expressed on a specific surface area basis, the Beta horizon was appreciably higher than the B₂ horizon of the same profile.

Clay mineral analyses indicated that the Beta horizons are characterized by a montmorillonitic suite more closely associated with the loess-influenced material in the upper part of the soil than an illitic clay suite which characterizes the drift material below.

The reaction ranged from pH 6.0 to 7.7 for most of the Beta horizons with the lower portion of the horizon having the highest pH.

Clay migration was suggested as the dominant process in the development of the Beta horizon and it was closely related to the discontinuity in moisture flow at the boundary of the coarser textured substrata and finer textured subsoil.

¹ Contribution from the Department of Agronomy, Illinois Agr. Exp. Sta. Urbana, and Soil Conservation Service, USDA. This work was submitted by the senior author as part of a thesis in partial fulfillment of the requirements for the Ph.D. degree in the Graduate College, University of Illinois. Presented before Div. V, Soil Science Society of America, Lafayette, Ind., Aug. 4, 1958.

² Soil Scientist, Soil Conservation Service, Urbana, Illinois, and Professor of Soil Physics, Department of Agronomy, University of Illinois, respectively.

Received for publication November 30, 1959. Accepted for publication March 18, 1960.