Two approaches have been used in measuring the rate of reaction between soil and phosphate. In one, a large volume of solution has been mixed with a small weight of soil; in the other, the volume of phosphate solution has been limited to that needed to moisten the soil to its field moisture content. Using a small volume of solution requires special techniques to measure the concentration in solution but has the advantage that the amount of phosphate in solution is small. As a result the amount adsorbed does not increase much after the first few minutes. Mathematical functions can then be used directly to relate the change in concentration to time. Using a large volume makes measurement of concentration easy but poses mathematical problems because both the concentration in solution and the amount adsorbed change simultaneously. It is not acceptable to relate concetration (or adsorption) to time without taking this into account. Published evidence indicates that differences between soils in the rate of reaction reflect differences in the kind of adsorbing surfaces present rather than merely differences in the amount of adsorbing surface.