Foliar potassium–calcium nutrition enhances fruit yield, quality and mitigates cracking in guava (Psidium guajava L.) under humid subtropical conditions

Introduction Guava ( Psidium guajava L .) productivity and fruit quality are frequently constrained by nutrient imbalances during reproductive development, leading to yield losses and physiological disorders such as fruit cracking. Potassium (K) and calcium (Ca) play critical roles in plant physiological regulation and cell wall stability, yet their coordinated effects on yield, fruit quality, and underlying trait relationships in guava remain insufficiently understood. Methods In this study, we evaluated the influence of foliar-applied K and Ca compounds on vegetative growth, leaf nutrient status, fruit yield, quality attributes, and fruit cracking in the high-yielding guava cultivar ‘VNR Bihi’ under subtropical humid conditions. A total of 13 treatments comprising different concentrations of potassium nitrate, potassium sulphate, calcium nitrate, and calcium sulphate, along with an untreated control, were applied at fruit set and one month thereafter. Results Foliar nutrient application significantly enhanced leaf growth, chlorophyll content, and post-treatment leaf K and Ca concentrations compared with the control. Yield parameters responded positively to nutrient supplementation, with potassium nitrate at 2.0% producing the highest fruit yield (41.44 kg tree⁻¹; 27.60 t ha⁻¹), accompanied by marked reductions in fruit drop and cracking. Fruit physical and biochemical quality traits, including fruit weight, total soluble solids, sugars, and ascorbic acid, were also improved by K and Ca treatments. Multivariate analyses revealed clear treatment-wise clustering based on fruit retention, drop, and cracking, while partial least squares regression demonstrated a strong predictive relationship between leaf physiological traits and fruit quality (R 2 = 0.877). Discussion Overall, the results indicate that foliar K and Ca application modulates guava yield and fruit quality through coordinated physiological and nutritional responses, providing a trait-based framework for reducing fruit cracking and enhancing productivity.