Greenhouse Evaluation of the Agronomic Potential of Urban Wastewater-Based Fertilizers: Sewage Sludge and Struvite for Lettuce Production in Sandy Soil

Environmental impacts of urban wastewater treatment plants (WWTPs) can be reduced by recovering nutrients and organic matter (OM) from their streams for agricultural use, decreasing dependence on conventional fertilizers. This study evaluated dehydrated sewage sludge (SS) as an organic amendment and the partial replacement of mineral P fertilizers in lettuce cultivation. Struvite, a byproduct of WWTPs, was also investigated as a sustainable P source. A 43-day greenhouse pot experiment assessed SS (12 t/ha) and struvite (at two P rates: 30 and 60 kg P2O5/ha), both alone and combined. SS significantly increased soil OM (p < 0.001), though long-term applications would be required to enhance this effect. The highest struvite rate (60 kg P2O5/ha) yielded the greatest extractable soil-P levels (150 ± 8.1 mg P2O5/kg), while its combination with SS further increased extractable P (>250 mg P2O5/kg), indicating a stable soil P pool. The highest plant dry biomass (8.9 ± 1.1 g, p < 0.05) also occurred under the highest struvite dosage. Complementary effects between SS and struvite were observed in foliar K, Ca, Mg, and S contents, although no significant interaction between both was found for P content. Adequate foliar P levels (0.40–0.52%) were achieved only in treatments containing SS, indicating its essential role in improving plant P nutrition.