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Mechanical and Mineralogical Subdivision of the Clay Separate of Soils¹

ABSTRACT

When clay from the A horizon of soils is separated so as to have an upper limit of particle size of 2u, it, at least when formed in humid climates, is practically devoid of primary minerals that weather easily, like the feldspars. Certain minerals, particularly quartz and muscovite, which are relatively stable toward chemical weathering agents, are probably often present in both primary and secondary form. Clay thus separated consists almost entirely of material which exhibits great resistance to further decomposition, although portions of it may be very reactive as regards base exchange and fixation of phosphorus and potassium. If the separation is made at the 5u point, appreciable amounts of feldspar and possibly other minerals that weather easily may still be present. The writers, therefore, favor separation at the 2u point.

When the clay separate is thus obtained from a soil which has been pretreated so as to remove organic matter, free iron oxides, and colloidal silica, it is possible to make a rather clean-cut subdivision of this separate at 0.2u into coarse and fine clay or colloid. The fine fraction usually contains nearly all of the true base exchange material and probably some quartz, muscovite, and other very stable minerals. The coarse fraction has altogether different chemical and physical properties, besides low base exchange capacity. It probably consists of muscovite and quartz and possibly talc, kaolinite, and other secondary minerals, which may be separated by means of heavy liquid specific gravity separations. For making the mechanical separation, a centrifuge equipped with an angle or conical head, direct reading speedometer, and time switch is essential. It takes about twenty-five washings to make a satisfactory separation. A special machine for thorough shaking at each washing is also essential.

¹ Contribution from the Department of Soils, University of Wisconsin, Madison, Wisconsin. Published with the permission of the Director of the Wisconsin Agricultural Experiment Station. This work was supported, in part, by a grant from the Wisconsin Alumni Research Foundation.

² Professor of Soils and Research Assistants, respectively.