Sustainable Valorization of Pulp and Paper Industry Sludge to Improve Alfalfa Nodulation in Acidic Soils

Industrial sludge from the pulp‐and‐paper sector is produced in large volumes and, when properly processed, can provide low‐cost alkalinity to correct soil acidity. We evaluated the agronomic potential of a phosphorus‐rich industrial sludge as a liming amendment in two acidic soils, Subeutric Luvic Brunosol (Soil A) and Distric Luvic Brunosol (Soil B), using Medicago sativa L. under controlled greenhouse conditions. A completely randomized pot experiment combined two soils with eight treatments (control; calcite at 0.45 and 2.5 t ha −1 ; dolomite at 0.41 t ha −1 ; a Ca–Mg–S product at 0.11 t ha −1 ; sludge at 1.31, 3.90, and 10 t ha −1 ). Measured variables included seedling emergence, stem and root growth, aboveground dry biomass, and nodulation (abundance, size class, position, and internal coloration). Because several variables were non‐normal, we analyzed treatment effects within soils using Kruskal–Wallis tests with Dunn's post hoc. Sludge increased soil pH after incubation and significantly enhanced nodulation and biomass in both soils. The intermediate dose (3.90 t ha −1 ) matched or exceeded dolomite without phytotoxic responses. Responses were soil‐specific and strongest in the Subeutric Luvic Brunosol. Overall, properly processed sludge performed as a viable partial liming alternative under greenhouse conditions. Field trials are warranted to assess multi‐season agronomic consistency and confirm environmental safety at operational scales.