Role of soil microbes in micronutrient solubilization

Micronutrients in soil include zinc, iron, copper, manganese, boron, molybdenum, chlorine and nickel. Their deficiency in soil may result from an inherently poor soil nutrient status, poor mobility or fixation in soil, reduced or poor solubility of the given chemical form of the nutrient and antagonistic interactions. The unavailable forms of these nutrients are solubilized by certain soil microbes through secretion of protons and release of chelated ligands through their oxidoreductive systems. Of all the micronutrients, Zinc (Zn) is the most deficient one; both in global and Indian soils. Zinc solubilizing bacteria (Bacillus sp.) present in soil secrete gluconic acid which increases the solubility of Zn through dissolution of insoluble and precipitated Zn compounds. Soil fungi, in general, solubilize zinc oxide preferentially followed by zinc carbonate and zinc phosphate. Several rhizobacteria secrete compounds that release siderophores like catechol, hydroxamate, pyoverdins which chelate and assimilate fixed iron from the soil. Some well known siderophore producing rhizobacteria include fluorescent pseudomonads, Arthrobacter, Aeromonas, Streptomyces, Bacillus and Nocardia. Aspergillus niger and Serpula himantioides were reported to solubilize different insoluble oxides of Mn. Rhizobacteria viz. Pseudomonas, Sphingomonas, Bacillus, Arthrobacter, Stenotrophomonas, etc. are active in copper solubilization. Several bacterial genera viz. Pseudomonas, Bacillus, Leptothrix, Citrobacter, Acidobacteria, Firmicutes, Nitrospira, Chromobacterium, Actinomyces, Azotobacter and Azospirillum are widely known for Mo solubilization and enhancing plant availability. Soil bacteria such as Gypsophila, Bacillus, and Microbacter can immobilize boron at pH 8 to 9.