Evaluating pulp and paper mill sludge as a potential amendment for soil acidity correction

Pulp and paper mills generate non-hazardous solid residues, including phosphorus-rich sludge, that can be valorised as soil amendments within circular economy strategies. This study evaluated the effectiveness of cellulose industry sludge to correct soil acidity in two acidic soils (Eutric Luvic Cambisol and Dystric Luvic Cambisol) under greenhouse incubation. Six sludge doses equivalent to 50%–200% of the liming requirement were applied, considering a neutralisation value of ∼50%. Soil pH(H2O) was monitored for 170 days, and magnesium (Mg), potassium (K), sodium (Na), calcium (Ca) and phosphorus (P) were analysed at the end of the trial. Sludge application consistently increased soil pH, reaching values above 7.0 only in the less acidic soil at the highest dose. Calcium increased markedly at the maximum rate in both soils, whereas Mg, K and Na remained largely unchanged. Phosphorus increased across treatments, reflecting both pH effects and the P contribution from the sludge. Overall, cellulose sludge effectively corrected soil acidity up to agronomically relevant pH levels (≈6.5), supporting its use as a liming alternative in systems where additional Ca–Mg fertilisation is not required. These findings provide practical guidance for dose selection based on a neutralisation value and reinforce the potential of industrial by-products in sustainable soil management.