Humic Acid-Amended Formulation Improves Shelf-Life of Plant Growth-Promoting Rhizobacteria (PGPR) Under Laboratory Conditions

Abstract

Plant growth-promoting rhizobacteria (PGPR) is a soil bacterium that positively impacts soil and crops. These microbes invade plant roots, promote plant growth, and improve crop yield production. Bacillus subtilis is a type of PGPR with a short shelf-life due to its structural and cellular components, with a non-producing resistance structure (spores). Therefore, optimum formulations must be developed to prolong the bacterial shelf-life by adding humic acid (HA) as an amendment that could benefit the microbes by providing shelter and carbon sources for bacteria. Thus, a study was undertaken to develop a biofertilizer formulation from locally isolated PGPR, using HA as an amendment. Four doses of HA (0, 0.01, 0.05, and 0.1%) were added to tryptic soy broth (TSB) media and inoculated with B. subtilis (UPMB10), Bacillus tequilensis (UPMRB9) and the combination of both strains. The shelf-life was recorded, and viable cells count and optical density were used to determine the bacterial population and growth trend at monthly intervals and endospores detection using the malachite green staining method. After 12 months of incubation, TSB amended with 0.1% HA recorded the highest bacterial population significantly with inoculation of UPMRB9, followed by mixed strains and UPMB10 at 1.8x107 CFUmL-1, 2.8x107 CFUmL-1and 8.9x106 CFUmL-1, respectively. Results showed that a higher concentration of HA has successfully prolonged the bacterial shelf-life with minimal cell loss. Thus, this study has shown that the optimum concentration of humic acid can extend the bacterial shelf-life and improve the quality of a biofertilizer.

Carrier Based Liquid Bioformulation of Salt-Tolerant Pgpr Bacillus Species for Prolonged Survivability

“…Arriel-Elias et al ( 2018) studied an experiment and concluded that a formulation containing glycerol, PVP, and molasses effectively boost the growth of bacterial strain species Pseudomonas fluorescens and Burkholderia pyrrocinia formulation throughout a 180-day shelf-life experiment, which demonstrates the effectiveness microbial activity pattern in the current study. A progressive increase in microbial activity was observed in a study conducted by Buraq et al (2023), that cell protectants such as humic acid significantly prolonged the plant growthpromoting bacterial shelf-life with minimal cell loss. The decrease in dissolved oxygen is due to increased O 2 demand for culture growth.…”