Effects of foliar potassium and calcium application on crop yield and physiological responses: a global meta-analysis

CONTEXT AND OBJECTIVE Foliar nutrient applications can rapidly correct physiological imbalances, thereby enhancing crop growth and yield performance, particularly under suboptimal environmental conditions. Here, we conducted a global multilevel meta-analysis to quantify yield responses to two widely used foliar fertilizers: potassium (FK) and calcium (FCa), and identify the production contexts associated with variation in their effectiveness. METHODS Our synthesis included 126 studies, yielding 379, 369, 440, 321, and 239 paired observations for crop yield, net photosynthetic rate (Pn), chlorophyll content, superoxide dismutase (SOD) activity, and malondialdehyde (MDA) concentration, respectively. Multilevel models accounted for study-level dependence and sampling covariance arising from shared controls, with categorical moderator analyses complemented by continuous meta-regression. RESULTS AND CONCLUSIONS The evidence base was concentrated largely in Asia, particularly China and India, and was dominated by a limited number of major crops. Across the pooled dataset, FK and FCa increased yield by 15.4% and 9.8%, Pn by 23.3% and 18.2%, chlorophyll content by 22.9% and 22.6%, and SOD activity by 20.6% and 33.2%, while reducing MDA concentration by 19.4% and 22.5%, respectively. Nutrient type did not significantly moderate these pooled responses. Yield responses also showed little evidence of systematic differences across broad crop groups, experimental settings, stress conditions, or nutrient-source formulations. Context dependence was selective: the categorical analysis showed a greater FK yield response in the lower initial soil-K category, while FK yield response was negatively associated with application rate and positively associated with application frequency; corresponding relationships for FCa were not significant. Supplementary evidence mapping associated FK responses mainly with ion and osmotic regulation, photosynthetic functioning, and carbon transport, whereas FCa responses were more closely aligned with Ca signaling, cell-wall metabolism, antioxidant defense, and membrane stability. SIGNIFICANCE Collectively, these findings show that FK and FCa produce broadly positive agronomic and physiological responses, while their variability is only partly explained by the production and management contexts evaluated. Initial soil K availability and FK application intensity provided the main context-related signals, supporting context-specific interpretation rather than universal foliar-fertilization recommendations.