Comparing the Effectiveness of two Innovative Slow-Release Fertilizers on Chemical Release Dynamics and Lettuce Plant Growth

Purpose: The use of conventional fertilizers raises concerns about nutrient losses, soil degradation, and non-renewable resource consumption. Waste-derived slow-release fertilizers (SRFs) may reduce these impacts by combining gradual nutrient release with residue valorization. However, complete Nitrogen, Phosphorous, Potassium (NPK) glass-based SRFs remain poorly explored, especially when nitrogen is supplied through an organic phase. We hypothesized that cold- and hot-consolidated SRFs, despite having the same raw materials and NPK content, would differ in nutrient release and plant response. Therefore, this study compared their NPK release, compliance with European SRF criteria, and effects on Lactuca sativa L. growth and physiology. Methods: The SRFs, characterized by crystalline or amorphous phases depending on processing temperature, were evaluated through solubility tests in citric and acetic acid to assess macro elements (N,P,K) release dynamics. Two repeated lettuce cultivation growth tests were conducted to measure agronomic parameters (fresh biomass, plant height, leaf area) and substrate chemical parameters and quality. Plants physiological status was assessed using the Nitrogen Balance Index (NBI). Results: Cold-SRF exhibited notably higher nitrogen release (75.9% at 90 days) than hot-SRF (< 5%). Phosphorus release in citric acid was comparable for both formulations (~ 80%), whereas in acetic acid it reached ~ 70% for cold-SRF but remained below 30% for hot-SRF. Potassium showed the opposite pattern, with hot-SRF releasing nearly completely (~ 100% in citric acid, ~ 90% in acetic acid) versus ~ 60% for cold-SRF in both media. Cold-SRF significantly improved plant biomass, height and leaf area (p < 0.05) compared with both the control and hot-SRF, and this agronomic performance was supported by enhanced NBI values. Conclusions: Cold-SRF provided a higher and more sustained overall Total Nutrient Release (TNR: 67.0% at 90 days) compared to hot-SRF (59.4%), driven primarily by the trends of N and P. This more sustained nutrient availability translated, in the 28-day lettuce trial, into superior plant performance: larger leaf area, greater plant height, higher above-ground biomass, and enhanced NBI values, indicating better nitrogen status and photosynthetic efficiency. Since the release remained active for up to three months while the agronomic trial already showed positive results within 28 days, cold-SRF also represents a promising option for low-maintenance fertilization strategies where fewer applications are desirable, reducing operational costs while maintaining adequate nutrition.