Due to the complexity of the plant-soil system, phosphorus (P) fertilizer efficiency will vary with the crop being grown, and the environment in which the crop is grown. In general, added P must increase the soil supply of P to roots, and the root system must be able to take advantage of the increase, for P fertilizer to be effective. Optimum placement of added P improves utilization by the crop root system, which sets the stage for maximum yields. Therefore, proper placement of fertilizer P in soil is just as important as choosing the correct amount to apply. The information in this presentation will provide a brief overview of soil P supply, factors affecting root growth, P uptake by roots, and finally, the relationship of these soil and plant processes to P fertilizer efficiency.