Organic–synthetic fertilization strategies and biochar amendment sustain yield and nutritional quality in Butternut Squash (Cucurbita moschata Duch.)

Fertilization strategies that sustain yield while enhancing nutritional quality remain a central challenge in vegetable production systems. This study was carried out to evaluate how the fertilization strategies, with or without biochar (0 vs. 7.5 t ha⁻¹), influence yield, biochemical, and elemental composition of butternut squash ( Cucurbita moschata Duch.). Fertilizers were full synthetic NPK (SF), poultry manure (PM), integrated mixture (PF; 60:40 PM: NPK), and unfertilized control (UC). Fertilization improved yield and key quality attributes. SF produced the highest yields, with PF and PM achieving statistically similar total yield and mean fruit weight. PF yield and fruit weight were only 4.71% and 7.56% lower than SF, respectively, and produced the greatest fruit number per plant. Total soluble solids (TSS) and dry matter increased with fertilization, with SF showing 29% greater TSS than UC. Biochar had a non-significant numerical reduction in yield and fruit weight, but biochemical profiles were context dependent, with a significant fertilizer × biochar interaction. Without biochar, carotenoids increased with a higher synthetic share, but the trend shifts under biochar. Polyphenols displayed a non-linear response: without biochar, PF increased polyphenols relative to SF, but biochar modified and sometimes reversed treatment rankings. Sugars reflected carbon allocation and dilution effects: glucose and fructose were highest in UC and generally lower under fertilization, while sucrose increased with fertilization in non-biochar soils; under biochar, SF ranked highest, and PF converged toward UC. Overall, an integrated mixture emerged as a balanced yield-quality approach while biochar benefits were context-specific and were targeted to low-input systems.