Landfill leachate is a complex, nutrient-rich waste stream with high potential for resource recovery through chemical precipitation. This study assessed the environmental safety and agronomic performance of struvite (MgNH4PO4·6H2O) and novel struvite–carnallite (KMgCl3·6H2O) mineral association, both being recovered from landfill leachate. A greenhouse experiment was conducted using lettuce grown in a highly P-fixing tropical soil Ultisol, comparing increasing doses of the recovered materials with a commercial NPK fertilizer. The double-dose struvite treatment (2S) corresponded to supplying twice the recommended P rate to assess the upper agronomic response. The 2S treatment produced the highest shoot dry mass to 13.7 g, representing a 105% increase relative to the NPK treatment, and resulted in the greatest foliar concentrations of P and N (6.55 and 50.4 g kg-1). The struvite–carnallite treatment (2SC) resulted in the highest K concentrations in the shoots and roots (74.7 and 19.9 g kg-1 ), demonstrating the agronomic relevance of carnallite formation during leachate treatment. Although Na accumulation reached 4337 mg kg−1 in the roots, no visual phytotoxicity or biomass reduction was observed. Concentrations of toxic metals in plant tissues (Cd < 0.19 mg kg−1; Pb < 1.13 mg kg−1) remained below regulatory thresholds, confirming the purity and environmental safety of precipitation-based mineral recovery. A strong linear relationship between soil P availability and shoot dry mass (R2 = 0.826, p = 0.002) indicates that the recovered minerals function effectively as slow-release P sources, capable of reducing P-fixation in weathered tropical soils. This study provides the first evidence of carnallite formation during nutrient recovery from landfill leachate, demonstrating that struvite-based fertilizers are sustainable, low-risk alternatives for tropical soil. These findings support waste valorization strategies to reduce dependence on imported mineral fertilizers in weathered soils.
