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Land application of alum sludge is a disposal alternative. A greenhouse experiment was conducted to determine (i) alum sludge effects on phosphorus (P) availability and inorganic P forms, and (ii) the relationship between inorganic P fractions and P availability. Alum sludge slurry (0, 4.45, 8.9, and 17.8 g solid kg−1 soil) was surface applied to wheat (Triticum aestivum L.) grown in a Yauhannah soil (fine‐loamy, siliceous, thermic Aquic Hapludults) at four P rates (0, 6.5, 11, and 22 mg kg−1 soil). After the first wheat crop, the sludge was mixed throughout the soil and a second crop grown. Surface‐applied sludge decreased wheat dry matter (DM) and P uptake. When incorporated, 4.45 g kg−1 alum decreased DM and P uptake. Phosphate application increased DM and P uptake of the first wheat crop, but had no effect on the second crop. Surface‐applied sludge increased Al‐P, Fe‐P, and Ca‐P in the 0‐ to 7.5‐cm soil depth, but not in the 7.5‐ to 15‐cm soil depth. Loosely boond‐P and Al‐P increased with P rate. Phosphorus uptake and DM of the first wheat crop were positively related to loosely bound‐P and Mehlich 1‐P. Dry matter of the second wheat crop was positively related to loosely bound‐P and Mehlich 1‐P and P uptake was positively related to loosely bound‐P. Aluminum‐P was negatively related to P availability indices. In alum sludge‐affected soils applied P is immobilized mainly as Al‐P, and Mehlich 1‐P and loosely boond‐P are reliable estimators of P availability.

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