Plants take up P from soil solution, so water-soluble P fertilizers are generally more effective than poorly soluble forms. The original sources of P used for agriculture were poorly soluble materials, including manures, bones, guano and phosphate rock. In contrast, highly soluble monocalcium phosphate, monoammonium phosphate and diammonium phosphate are the major compounds present in modern, manufactured solid fertilizers containing water-soluble P. This paper describes how the water-soluble P fertilizers are made, the dissolution of P in the granules of the fertilizers, the reactions of the fertilizer solution with the soil as P moves out of the granule into the soil, the agronomic effectiveness of the fertilizer in the year of application, and in the years after application (residual value). [Article copies available for a fee from The Haworth Document Delivery Service: 1–800–342–9678. E-mail address: [email protected]]