As the adsorptive ability of soil for cation was considered to be governed by two factors, i.e., its capacity and intensity, the author tried to estimate the intensity of soil to hold cation by the following two methods. A soil was equilibrated with a N- or N/10-salt solution of several cations by leaching; and then the amount of the cation adsorbed from the solution by the soil was determined. As the dilution of salt solution always decreased the amount of the adsorbed cation in widely different extent, the rate of this decrease was assumed to a index of intensity to hold the exchangeable cation against hydrolysis. Moreover, when a soil was equilibrated with several salt solutions, pH of which was 7,6,5 or 4,the decrease of pH also brought about a variable decrease of the cation adsorption. The rate of this decrease was to be a index of intensity to hold the exchangeable cation against acidification. Two soils having a remarkably different nature (table 1) were selected, and the above mentioned intensities were estimated for Ca and NH_4 ions. As shown in table 2,3,4 and 5,the intensity for Ca ion was not parallelled with that for NH_4 ion. This discrepancy was more cleary shown from the results of selective adsorption in a Ca and NH_4 mixed solution (table 6 and 7). To interpret the above facts, the author has considered as follows : The soil humus has a tendency to adsorb Ca much more strongly than NH_4 ion. It may, however, be not a special nature of humus but a common tendency shown by ion exchange substances. The mineral clay, on the other hand, has a specific property to adsorb NH_4 ion to a remarkable extent. The presence of its crystal structure is assumed to be a cause of this specificity.
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