Abstract Water treatment residual (WTR) is a by-product generated from drinking water treatment facilities that often contains considerable amounts of aluminum (Al) or iron (Fe). Application of WTR to agricultural land can cause deficiencies in soil phosphorus (P). Amendment of WTR with P before application to agricultural land may eliminate the problem of P deficiencies in plant growth. The objective of this field study was to estimate the amount of P required to make WTR a supplier of P to plants. A Fe-based WTR that contained a cationic polymer was collected from a sand-bed lagoon of a Connecticut water treatment facility in the spring of 1996 and 1997. The WTR was amended with P at rates of 19.4, 38.8, 62.0, and 77.4 g P kg−1 WTR in 1996, and 0.0, 4.9, 9.7, 14.6, 19.4, 38.8, and 62.0 g P kg−1 WTR in 1997. Fertilizer P was applied at rates of 0–486 kg P ha−1 for the calculation of fertilizer P equivalents of the P-amended WTR. Sorghum-sudangrass (Sorghum sudanenese “Ciba FP-5”) was planted in 1996 and ...