Sewage sludges have been used for many years as sources of P for agricultural crops, but there is a lack of information regarding the proportion of sludge P that can be used by crops. The aim of this work was to assess the importance of soil available P and sludge origin on the utilization of sludge P by plants. First, the changes in soil P isotopically exchangeable within 1 min (E1min) were measured in incubated soil‐sludge mixtures using two soils and four sludges. Then, the uptake of sludge P by ryegrass (Lolium perenne L.) was measured on the same sludge‐amended soils. The application of sludges increased E1min to values lower or equal to those obtained following the application monocalcium phosphate. Similarly the utilization of sludge P by ryegrass was systematically lower than the utilization of P derived from a water‐soluble fertilizer. In both soils, the lowest utilization of sludge P was observed for the two FeSO4 flocculated and anaerobically digested sludges, while the primary sludge and the aerobically digested sludge released somewhat higher quantities of P to ryegrass. In the clayey soil, the amount of sludge P taken up by the crop was significantly related both to the sludge and soil available P content, whereas no such relation was observed in the loamy soil because of its high available P content. The origin of the sludge and the soil available P content must therefore be taken into account when advising sludge application to crops to adjust P inputs to plant needs.