Boron toxicity thresholds in field-grown maize fertilized with ulexite incorporated into a clayey Oxisol

Boron (B) management in tropical maize is constrained by the narrow range between deficiency and toxicity, particularly when soluble sources are applied in-furrow in highly weathered Oxisols. We evaluated field-based soil and foliar B toxicity thresholds for maize ( Zea mays L.) fertilized with broadcast-incorporated ulexite (10% B) in a clayey Oxisol (54% clay). Five B rates (0, 3, 9, 27, and 81 kg B ha − 1 ) were tested in a randomized block design during the 2023–2024 growing season in Brazil. Grain yield reached a maximum of 6,391 kg ha at 31 kg B ha − 1 and declined by 10% at 69 kg B ha − 1 , establishing the agronomic toxicity threshold. This threshold corresponded to 7.7 mg B dm − 3 in soil and 118 mg B kg − 1 in diagnosis leaves. Increasing B supply promoted B accumulation in soil and plant tissues, increased the flavonoid index, and reduced chlorophyll, nitrogen balance index, net photosynthesis, and grain yield; malondialdehyde (MDA) followed a significant U-shaped response, with higher concentrations at the lowest and highest B rates than at intermediate, agronomically adequate doses. Principal component analysis confirmed coordinated physiological and nutritional responses to excessive B supply. These findings provide field-based toxicity benchmarks specific to maize fertilized with broadcast-incorporated ulexite in a clayey tropical Oxisol, supporting safer corrective B fertilization strategies.