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Procedure for Special Type of Mechanical and Mineralogical Soil Analysis¹

ABSTRACT

In certain types of mechanical and mineralogical soil analysis, it is essential that the colloidal binding material consisting of organic matter, free iron and aluminum oxides, and colloidal silica be removed so as to get complete dispersion of the soil particles. The organic matter is commonly removed by means of hydrogen peroxide. Previously, the use of hydrogen sulfide had been suggested for removing the free iron oxide. Recently, it was found that the addition of oxalic acid to a soil suspension containing sodium sulfide, producing nascent hydrogen sulfide, is many times more effective than ordinary hydrogen sulfide for this purpose. With this improved procedure, it is possible to dissolve the free iron oxide in about ten minutes without greater acidification than pH 3.5. Free alumina is also dissolved. Boiling for a few minutes right after the addition of the sodium sulfide, which is quite alkaline, serves to dissolve colloidal silica. The amounts of these constituents dissolved are later added to the clay fraction. After this pretreatment and subsequent saturation with sodium, the soil is in ideal condition for mechanical and mineralogical analysis.

Adoption of 2u as the upper limit of clay particle size is favored because clay thus separated consists largely of secondary minerals along with a few very resistant primary minerals like quartz and muscovite. Feldspars, the major group of primary minerals, are practically extinct below 2u. Separation at 5u does not allow this simple and desirable assumption. A centrifuge equipped with special centrifuge tubes, direct reading speedometer, and automatic time switch, and a special shaking machine are desirable equipment for making these analyses.

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. The writers are indebted to S. Kliman for making the drawings of figures 1, 2 and 3.