Phosphorus (P) immobilization has potential for reducing diffuse P losses from legacy P soils to surface waters and for regenerating low-nutrient ecosystems with a high plant species richness. Here, P immobilization with iron oxide sludge application was investigated in a field trial on a noncalcareous sandy soil. The sludge applied is a water treatment residual produced from raw groundwater by Fe(II) oxidation. Siliceous ferrihydrite (Fh) is the major Fe oxide type in the sludge. The reactive surface area assessed with an adapted probe ion method is 211–304 m2 g−1 for the Fe oxides in the sludge, equivalent to a spherical particle diameter of ~6–8 nm. This size is much larger than the primary Fh particle size (~2 nm) observed with transmission electron microscopy. This can be attributed to aggregation initiated by silicate adsorption. The surface area of the indigenous metal oxide particles in the field trial soils is much higher (~1100 m2 g−1), pointing to the presence of ultra-small oxide particles (2.3 ± 0.4 nm). The initial soil surface area was 5.4 m2 g−1 and increased linearly with sludge application up to a maximum of 12.9 m2 g−1 when 27 g Fe oxides per kg soil was added. In case of a lower addition (~10–15 g Fe oxides per kg soil), a 10-fold reduction in the phosphate (P-PO4) concentration in 0.01 M CaCl2 soil extracts to 0.3 µM was possible. The adapted probe ion method is a valuable tool for quantifying changes in the soil surface area when amending soil with Fe oxide-containing materials. This information is important for mechanistically predicting the reduction in the P-PO4 solubility when such materials are used for immobilizing P in legacy P soils with a low P-PO4 adsorption capacity but with a high surface loading.