The discovery that soil colloids are predominately composed of clay minerals has set in motion a number of researches on the chemical and physical properties of specimens of the several recognized types of such minerals. This investigation is concerned with the effect of the mineralogical and chemical nature of colloids on the availability to plants of replaceable calcium.It has been demonstrated that replaceable cations are an important source of plant nutrients, and that plants obtain these nutrients by cation-exchange reactions. The influence of the nature of the soil colloid in these exchange reactions is not well understood, however, and it seems likely that quantitative measurements of this factor will aid in the laboratory evaluation of soil fertility. The object of this investigation was to compare the manner in which different types of colloids release their replaceable Ca to growing plants and to solutions of electrolytes, and to measure the effect of percentage Ca saturation of the various colloids upon the magnitude of this release.