Potassium Humate and Bacillus aryabhattai Modulate Osmotic and Oxidative Homeostasis and Promote Cowpea Yield Under Drought

Global climate change has intensified prolonged droughts, making the development of sustainable agricultural technologies essential. Although potassium humate (KH) and Bacillus aryabhattai (BA) are recognized individually as effective biostimulants, their joint application represents an innovative, unexplored complementary strategy to mitigate drought stress in cowpea (Vigna unguiculata L. Walp.). This study aimed to evaluate the effects of these bio-inputs on mitigating water stress in the cowpea (Vigna unguiculata L. Walp.) cultivar ‘BRS Verdejante’. The experiment was conducted under greenhouse conditions using a completely randomized design in a 2 × 8 factorial arrangement, with two water regimes (100% and 50% crop evapotranspiration—ETc) and eight biostimulant combinations of KH and BA. While BA alone (T4) intensified osmotic adjustment via proline accumulation and APX activation, the combined treatment T5 (10 mg kg−1 soil KH + 4 mL kg−1 seed BA containing 1 × 108 CFU mL−1) achieved the best overall performance under drought (50% ETc). T5 effectively enhanced superoxide dismutase activity, preserved leaf relative water content, and reduced cell membrane electrolyte leakage by 42.6% compared to untreated stressed plants. These physiological adaptations protected reproductive organs, resulting in a 51.6% increase in pod number per plant under drought. The strategic co-application of low-dose potassium humate (10 mg kg−1 soil) and B. aryabhattai (4 mL kg−1 seed) effectively regulates osmotic and oxidative homeostasis, maintaining pod formation (+51.6%) and securing cowpea grain yield per plant under drought stress, thus serving as a promising strategy to mitigate drought stress in cowpea under controlled greenhouse conditions.