Separation of the phosphorus (P) contributions from soil, fertilizer and biosolids to plants has not been possible without the aid of radioisotopes. Dual labelling of soil with 32P and fertilizer with 33P isotopes has been used to partition the sources of P in maize (Zea mays) shoots and in soil P pools. Biosolids containing 4.1% P that had been prepared using Fe and Al were applied to a Kurosol soil from Goulburn, NSW, Australia. The biosolids were applied at five rates up to 60 dry t/ha with and without P fertilizer. Phosphorus derived from fertilizer was determined directly with33 P and that from soil by32 P reverse dilution. Phosphorus derived from biosolids was estimated as the difference between total P and that derived from the soil plus fertilizer calculated from isotope data. Yield and P content of maize shoots increased linearly with the rate of biosolidsapplication. The proportion of P in the plant derived from biosolids also increased with application rate up to 88% for the soil receiving biosolids at 60 dry t/ha with no fertilizer. The corresponding value with fertilizer applied at 80 kg P/ha was 69%. The proportion of P in the maize shoots derived from soil and fertilizer decreased as biosolids application rate increased. Soil total P, bicarbonate extractable P, Al-P, Fe-P and Ca-P increased with biosolids application rate. The increase in plant P uptake and in bicarbonate extractable P in the soil shows that biosolids P provides a readily available source of P. A decrease in uptake of fertilizer and soil P with increasing biosolids application is attributed to the decrease in the proportion of P from these sources in the total pool of available P, rather than to immobilization of P by Fe and Al in the biosolids.