In this paper is dealt with the exchange capacity of soil colloidal clay (<1μФ) and the rate of its decomposition by HCl of 1.10 sp. gr., as related to the climatic soil types. The soils examined are : (1) Thirteen samples of three profiles in Manchuria. (a steppe-, a brown forest-, and a podzol soil) (2) Seven surface soils of different mother rock in North-Kyushu. (3) Nine surface soils of different mother rock in Okinawa island. (4) Three surface soils in humid tropics, -Hongkong, Johore in Malay and Colombo. The total exchange capacity was ascertained by leaching with slightly ammoniacal ammonium acetate solution (ca. N) and determining the absorbed nitrogen by distillation with MgO. The total exchange capacity is expressed as M.E. per 100 gram of dry colloidal clay. A weighed portion of colloidal clay was treated with HCl of 1.10 sp. gr. at room temperature and after standing over night, the residue was filtered off and washed. The other weighed portion was digested one hour with HCl of 1.1O sp. gr. at boiling temperature with reverted cooler, then the residue was filtered off and washed. The exchange capacity of these two residues was determined by ammonium acetate method. Both the average values of total exchange capacity and the reduction in exchange capacity of the acid-treated residues as percentage of the total, and also the silica-alumina ratio are recorded in table I. Table I. Average exchange capacity of colloidal clay and its percentage reduction by treatment with HCl. [table] From the data in table I the following summaries are given : 1) The exchange capacity of manchurian soil colloidal clay is extremely high and that of humid tropics is the lowest, and those of North-Kyushu and Okinawa island lie between them. 2) The exchange capacities of each soil colloidal clays are not much reduced by the action of hydrochloric acid at room temperature. 3) The exchange capacity of residue, boiled one hour with HCl.1.10 sp. gr., of manchurian soil colloidal clay is reduced to the highest degree, and that of humid tropics to the least degree. 4) The exchange capacity of residue itself, boiled with HCl, of manchurian soil colloidal clay is the greatest, and its magnitude is greater than that of total capacity of the tropics. 5) The presence of exchange complexes other than crystalline clay minerals, for example, , , mixed gels", appear to be responsible for a considerable portion of that of soil colloidal clay, and the proportion of mixed gels contented, is the highest in manchurian soil colloidal clay and the lowest in that of the humid tropics.
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