Integrated application of <i>Bacillus subtilis</i>, boron, and organic amendment improves yield, fruit quality, and nutrient status of red-fleshed guava

Red-fleshed guava (<i>Psidium guajava</i> L.) is a high-value tropical fruit, but its productivity and fruit quality are often limited by nutrient imbalance and soil degradation under tropical field conditions. This study aimed to evaluate the integrated effects of foliar-applied <i>Bacillus subtilis</i>, boric acid, and fermented cassava-based organic amendment on growth, yield, fruit quality, and nutrient dynamics of red-fleshed guava. A two-season field trial (2023–2024) was conducted in Nakhon Ratchasima Province, Thailand, using a randomized complete block design with six treatments. The integrated treatment (T6) significantly improved plant height (184.05 ± 10.24 cm), fruit weight (270.26 ± 19.84 g), and yield (6.69 ± 0.54 kg/plant) compared to the control. Fruit quality was enhanced through higher firmness and desirable °Brix levels under boron-containing treatments. Foliar nitrogen (2.25%), phosphorus (0.32%), and potassium (1.82%) concentrations were highest in T6-treated plants. Post-harvest soil analysis revealed elevated levels of available phosphorus (16.5 mg/kg) and exchangeable potassium (111.5 mg/kg) in T6 plots. Pearson correlation revealed strong positive relationships between leaf nutrient levels and yield components. The results confirm that integrated use of <i>B. subtilis</i>, boric acid, and organic amendment improves both plant productivity and soil nutrient status. This integrated foliar-and-soil nutrient management strategy is recommended for semi-organic guava orchards in tropical regions.