Introduction: Water scarcity, drought stress, and nutrient limitations—particularly phosphorus (P) deficiency—are among the most serious constraints affecting crop production in arid and semi-arid regions. In recent decades, the sustainability of agricultural systems has been increasingly threatened by the overuse of chemical fertilizers, rising production costs, and environmental degradation. In this context, attention has shifted toward the use of natural organic amendments as alternatives or supplements to mineral fertilizers. Leonardite, a highly oxidized form of lignite and an abundant source of humic and fulvic acids, has been reported to improve soil physicochemical properties, enhance nutrient availability, and stimulate plant growth. Considering these benefits, this study was conducted to evaluate the combined effects of triple superphosphate (TSP) and leonardite on yield, yield components, oil content, and oil yield of black seed (Nigella sativa L.) under rainfed conditions, and to investigate the correlations among the studied traits.

Materials and Methods: The field experiment was carried out during the 2021–2022 cropping season in the Parchin region of Ardabil province, Iran. A randomized complete block design (RCBD) with three replications was employed. Treatments included three levels of triple superphosphate (25, 50, and 100 kg ha⁻¹) and two levels of leonardite (100 and 200 kg ha⁻¹), applied individually and in combination (e.g., P25H150, P50H100, and H200). Key agronomic and biochemical traits measured were seed yield, 1000-seed weight, number of secondary branches, number of capsules per plant, number of seeds per capsule and per plant, stem diameter, oil percentage, oil yield, and seed weight per plant. Statistical analyses were performed using SPSS software, including ANOVA and Duncan’s multiple range test at the 5% level, while figures were prepared using Excel.

Results and Discussion: Significant differences were observed among treatments for most of the evaluated traits. The combined treatment P25H150 (25 kg ha⁻¹ TSP + 150 kg ha⁻¹ leonardite) recorded the highest improvements in seed yield, oil content, and oil yield compared with the control and full phosphorus application (P100). The treatments H200 and P50H100 also exhibited notable enhancements in growth and yield parameters. The positive response of black seed to leonardite is attributed to its ability to enhance soil structure, water retention, and nutrient uptake under rainfed and low-input conditions. Correlation analysis revealed strong and positive relationships between seed yield and yield components such as number of capsules per plant, number of seeds per capsule, 1000-seed weight, and stem diameter. Moreover, oil yield was highly correlated with seed yield, oil percentage, and seed weight per plant, indicating that improving vegetative growth and reproductive efficiency simultaneously contributes to higher productivity.

Conclusion: The findings demonstrated that integrating leonardite with reduced phosphorus fertilizer rates significantly enhances both quantitative and qualitative traits of black seed. The P25H150 treatment, in particular, achieved superior performance while reducing chemical fertilizer input by 75%, offering an eco-friendly and cost-effective alternative to full mineral fertilization. Thus, leonardite—either alone or in combination with moderate levels of P fertilizer (H200, P25H150, or P50H100)—can be considered a sustainable nutrient management strategy for medicinal and oilseed crops under dryland farming. By improving nutrient-use efficiency, maintaining soil fertility, and minimizing environmental impacts, leonardite application represents a practical step toward sustainable and resilient agricultural production systems under the constraints of water limitation and climate change.