The mineral constituents of the colloidal fraction of soils

Previous work having shown that the base exchange property of the mineral matter of soil colloids is probably due largely to one compound and that the chemical composition of this compound is A1(2)O(3.4) SiO(2).XH(2)O, it seemed reasonable to believe that, besides this compound, mineral soil colloids consist largely of free silica, free alumina, free titanium oxide, free iron oxide, and possibly kaolinite. From a total analysis of soil colloids, an alkali-soluble alumina and silica determination, and a base exchange capacity determination, it should he possible, by calculation, to formulate a fairly accurate picture of the mineralogical composition of soil colloids. It was the purpose of the present study to investigate this possibility. In order to make the calculation possible, it was necessary to determine accurately the base exchange capacity of the base exchange compound. For this purpose samples of the purified base exchange compound were prepared from three different bentonites and the exchange capacities of these samples were determined by both the exchange method and the titration method. One of the samples gave exactly the same exchange capacity value by the two methods, and it was assumed that this sample was probably the purest. The exchange value of this sample was used, therefore in calculations of the mineralogical composition of the soil colloid. A sample of soil colloid was prepared from Colby silt loam subsoil and this was subjected to a base exchange capacity determination and also to the other analyses referred to. From the base exchange capacity determination, it appears that this soil colloid consists of a little more than 50% of the base exchange compound. The amounts of free silica and alumina are rather small. There exists an excess of combined silica and alumina over that required for the base exchange compound. This excess of these two oxides was not in the ratio found in kaolinite, and since other evidence did not indicate the presence of kaolinite, it was assumed that this combined silica and alumina were probably present in the form of chlorites. Chlorites are found in abundance in the weathered portion of soils, and their presence in the colloid would also account for the appreciable amount of magnesia found. A portion of the iron oxide was probably also present in the chlorites. On the basis of this assumption, 30 to 40% of the soil colloid consisted of chlorites.