This paper summarizes recent work on the slow reactions between soil and phosphate or molybdate. These slow reactions are important in determining long-term effectiveness and residual value. It was found that the rate of the reaction could be described by a simple equation, and that the rate was very sensitive to temperature, but not greatly affected by differences between soils or by a range of water contents. The proportion changed to an ineffective form was independent of the level of application. The differences between adsorption and desorption of phosphate could be explained by the continuing slow reactions.