Biochar–NPK–Seaweed Integration as a Sustainable Strategy to Boost Productivity of Spearmint in Sandy Soils

Sandy soils in arid and semi-arid regions of Egypt are characterized by poor structure, low fertility, and a limited capacity to retain irrigation water, which collectively constrain nutrient availability and crop productivity under arid conditions. Despite these limitations, improving the performance and sustainability of sandy soils has become essential to meet increasing agricultural demands. Therefore, this study aimed to evaluate the individual and combined effects of biochar, mineral NPK fertilization, and seaweed extract on the growth performance, biomass production, nutrient status, and overall productivity of spearmint ( Mentha spicata L.) cultivated in sandy soil. Field experiments were conducted over two successive growing seasons (2024 and 2025) at the Agricultural Research Station, Al-Marashda, Qena Governorate, Egypt, using a split-plot design with biochar application (0 and 12.5 ton ha −1 ) as the main factor and foliar growth stimulants (control, NPK, NPK + 2 mL L −1 seaweed extract, and NPK + 4 mL L −1 seaweed extract) as sub-factors. Results revealed that biochar application significantly improved all vegetative growth parameters, herbage fresh and dry yields, essential oil percentage, oil yield per plant, photosynthetic pigment concentrations, and leaf N, P, and K contents compared with untreated soil. Foliar application of NPK fertilizer, particularly when combined with seaweed extract, further enhanced plant performance. The greatest improvements across all measured traits were consistently obtained from the integrated application of biochar at 12.5 ton ha −1 combined with foliar spraying of NPK (5 g L −1 ) and seaweed extract 4 mL L −1 . This treatment produced the highest biomass accumulation, essential oil yield, chlorophyll content, and nutrient uptake during both seasons. The findings conclude that integrating biochar with balanced mineral fertilization and natural biostimulants represents an effective and sustainable strategy for improving productivity and essential oil yield of spearmint grown in nutrient-poor sandy soils.