The present paper deals mainly with the characteristics of isotherm of ammonium adsorbed by several typical soils and clay minerals and cation-exchange resin from binary solution of NH₄⁺-Ca⁺⁺ chloride and incidentally with the selectivity of the soils for ammonium ion. Some information of soil samples are given in Table 1. Binary solution were prepared with NH₄Cl at 8 concentrations ranged from 0.002 N to 0.02N and various amounts of CaCl₂ were added in order to keep the total ionic concentrations of binary solution at 0.02N as are showed in Table 2.The ratio for soil or kaolinite to binary solution in the experiment was 1:20, and that of bentonite or resin to the solution was 1:100. Each sample was put in a definite volume of binary solution and shaken for half an hour, and then filtrated. In filtrate, the ammonium ion was determined by semi-micr,odistilled method and calcium ion was sometimes determined by EDTA volumetric method. The difference between initial and final concentrations of NH₄⁺ and Ca⁺⁺ in solution is the amount of NH₄⁺ and Ca⁺⁺ adsorbed.The adsorbed ammonium by soil was positively correlated with the NH₄⁺ concentration in solution. The equation (1) or (2) derived from Ratio Law gave the beat results as are showed in Fig.1 and Table 3-5.The increment of adsorbed ammonium of soil samples removed the organic matter and samples after submerging were not related to the ammonium ion concentration of binary solution, while the increment of adsorbed ammonium of soil samples removed free iron oxides was positively correlated with the ammonium ion concentration of binary solution. The correlation coefficient (r) was 0.950^*** for yellow brown earth 0.993^*** for red earth and 0.950^*** for laterite respectively. The authors suggest that the ezchange sites released from soil samples after removed organic matter and treatwent under submerged condition have a strong affinity for ammonium ion. Therefore there were no difference between the increments of ammonium adsorbed in binary solution with lower concentration and higher concentration of ammonium. however, the exchange sites released by removed free iron oxides were oppsite to those released by removed organic matter and treatment of submergence. In addition, in accordence with the results of Table 8 and with the released amount of adsorbed ammonium the relative affinity of soil samples for ammonium ion may be as follows:Yellow brown earth ≥ Red earth ≥ Laterite.