SummaryAt low levels of annual rainfall, leaching of bases with falling percentage saturation follows a simple physical pattern, but as rainfall rises above a level sufficient for reasonable plant growth, intrusion of the biological factor leads to a recirculation of bases and even to an accumulation in the solum with the result that percentage saturation rises once more to an optimum at about 46 in. rainfall and falls away with further increase in rainfall.These changes in percentage saturation are independent of the base‐exchange capacity of the parent material, and the total exchangeable bases in a soil are determined by the base‐exchange capacity of the parent material and the rainfall régime to which it has been subje