Major Role

Exogenous application of soluble zinc sources, similar to fertilizer application, has been advocated to various crops. About 96 to 99% of the applied zinc is converted into different type of insoluble forms depending upon the soil types, physico-chemical reactions of the soil. The solubility of zinc is highly dependent on soil pH and moisture. Zinc occurs in soil as ores of augite, biotite, hornblende, olivine and sphalerite. However, availability of zinc from these sources is guided by many factors among which biochemical actions of rhizosphere microorganisms play an important role in converting such unavailable sources into available ones (Bhupinder et al., 2005). The zinc thus made unavailable can be reverted back to available form by application of bacterial strain capable of solubilizing it. Bacteria are known to immobilize zinc metal by precipitation and adsorption. The capability to dissolve immobilized zinc i.e. zinc carbonate, zinc oxide and zinc phosphate in appreciable quantity is not universal feature amongst the cultivable bacteria on soil surface. Few zinc solubilizing bacterial genera viz., Thiobacillus ferrooxidans, Thiobacillus thiooxidans, Acinetobacter sp., Bacillus sp., Gluconacetobacter sp., Pseudomonas sp. and facultative thermophilic iron oxidizers were reported as zinc solubilizers.

plant growth and development

These bacteria improve the plant growth and development by colonizing the rhizosphere and by solubilizing complex zinc compounds into simpler ones, thus making zinc available to the plants. Zinc solubilizing microorganisms solubilize zinc through various mechanisms, one of which is acidification.