Agronomic Performance of Solid and Liquid Digestate from Neem Fruit Pulp (Azadirachta indica A. Juss) on Growth and Yield of Lettuce (Lactuca sativa L.) under Tropical Conditions

Abstract

Background and Aims: Anaerobic digestion digestates represent a promising organically derived alternative to mineral fertilizers in sustainable agriculture; however, their agronomic efficiency is highly dependent on post-digestion management practices. This study compared the agronomic performance of the solid and liquid fractions of digestate derived from the anaerobic digestion of neem fruit pulp (Azadirachta indica A. Juss) on lettuce (Lactuca sativa L.). Study Design: A randomized complete block design (six treatments, three replications) was established using an isonitrogen application rate of 100 kg N ha⁻¹ (i.e. all treatments supplying the same total nitrogen). Place and Duration of Study: Agronomic Experimental Station of Lomé (SEAL), Togo, between July 2025 and August 2025. Methodology: Treatments consisted of an unfertilized control (T0), mineral NPK 15-15-15 fertilizer (TN), ENPRO compost (TC), solid digestate (TSD), liquid digestate (TLD), and a mixed digestate combining 50% solid and 50% liquid fractions (TM). Growth parameters recorded included leaf number, leaf spread diameter, leaf color chart index, and final marketable yield. Treatment effects were assessed using a linear mixed model, with means separated by Tukey's HSD test (P < 0.05). Results: Phase separation resulted in contrasting chemical profiles: the liquid digestate exhibited a C/N ratio of 7.9, conducive to rapid ammonium nitrogen release, whereas the solid digestate displayed a C/N ratio of 18.2, associated with slower nitrogen mineralization. Mineral fertilizer (TN) achieved the highest yield (14.93 t ha⁻¹), followed by liquid digestate (TLD) at 13.70 t ha⁻¹, a performance not statistically different from TN (P > 0.05), while the overall treatment effect on yield was highly significant (P < 0.001). Compost (TC) and mixed digestate (TM) yielded intermediate and statistically similar results, while solid digestate (TSD) recorded the lowest performance among fertilized treatments (9.00 t ha⁻¹), corroborating the mismatch between its slow nitrogen release kinetics and the short growing cycle of lettuce. Conclusion: The liquid fraction of neem fruit pulp digestate represents an organically derived alternative whose agronomic efficiency is statistically comparable to that of mineral NPK fertilizer under the conditions of this trial. These findings are based on a single short-cycle trial in which only nitrogen was standardized across treatments, and therefore warrant confirmation under multi-season conditions.