Optimization of appropriate boron fertilization application range for cotton (Gossypium hirsutum L.) based on two varieties in boron-deficient soils

Boron (B) deficiency critically constrains cotton productivity in China’s major cotton-growing regions, where soil available B levels widely fall below the sufficiency threshold (< 0.5 mg kg ^− 1 ). This multi-site study (2020–2021) quantified the optimal B application ranges for two regionally adapted cultivars (CCRI 425 and Siza 3) through field trials and column experiments across three B-deficient sites in Jiangsu Province. Quadratic regression modeling of seed cotton yield responses identified distinct optima: 1.90–2.36 kg B ha ^− 1 for CCRI 425 and 2.05–2.36 kg B ha ^− 1 for Siza 3, achieving yield increases of 14.7–25.9% compared to B-free controls. Within these ranges, key agronomic traits demonstrated peak performance: boll shedding rates decreased by 11.3–42.0%, boll number increased by 12.4–22.0%, and boll size expanded by 16.9–30.8%. The coefficient of variation for boll number (4.2–6.6%) substantially exceeded that of boll weight (0.9–2.2%), identifying boll number as the primary yield determinant. Vertical canopy analysis revealed that middle strata (FB5–8) accounted for 48.2% of the total yield and was more sensitive in response to B fertilizer, likely attributable to restricted B phloem mobility. These findings provide a scientific framework for precision B management, balancing yield maximization with nutrient stewardship in B-deficient cotton systems.