Isolation and characterization of calcite-solubilizing plant growth-promoting bacteria and their effects on maize in calcareous soil under controlled conditions

Calcite-solubilizing bacteria (CSB) are emerging as novel agents for agricultural applications. Twenty-four CSB strains were isolated from marble slurry dump yard soil and characterized morphologically and biochemically. Plant growth-promoting traits included IAA production (23 strains), phosphorus solubilization (20), ammonia production (23), HCN production (21), siderophore production (17) and potassium solubilization (22). Amplified ribosomal DNA restriction analysis (ARDRA) revealed considerable phenetic variation among isolates. In pot experiments, CSB inoculation enhanced maize growth in calcareous soil. Two isolates, CSB-7 and CSB-9, which gave the most consistent improvement, were identified by 16S rRNA gene sequencing as Bacillus sp. and Exiguobacterium sp., respectively. Inoculation increased chlorophyll content, root and shoot length and biomass relative to uninoculated controls. Both isolates solubilized CaCO₃ and showed several in vitro growth-promoting traits; soil properties and underlying mechanisms were not measured. These findings suggest CSB from marble slurry environments merit further field evaluation as candidate inoculants for calcareous soils.