Organic and inorganic acids produced by PSM dissolve the insoluble soil phosphates by chelation of cations and competing with phosphate for adsorption sites in the soil. The hydroxyl and carboxyl groups of the acids chelate the cations bound to phosphate, thereby converting it into soluble forms. These acids may complete for fixation sites of Al and Fe insoluble oxides, on reacting with them, stabilize them, and are called “chelates”. 2-ketogluconic acid is a powerful chelator of calcium. Production of inorganic acids, such as sulphidric, nitric,and carbonic acid,has been reported. Nitric and sulphuric acids react with calcium phosphate and convert them into soluble forms.

There are many metabolites of microbial origin which may solublize and chelate or complex cations. Amino acids and low molecular weight organic acids can improve the solubility of cations as well as anions like P and B. Biofertilizers having acid producing bacteria can help to improve availability of nutrients.

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https://books.google.com.sa/books?id=GVmZBQAAQBAJ&pg=PA315&lpg=PA315&dq=Chelating+agent+can+be+used+with+bio-fertilizers+application&source=bl&ots=JuLzTJCJs3&sig=ACfU3U2A1aGffQp2vXgukQHu5OV-yoQSQw&hl=en&sa=X&ved=2ahUKEwjNkoODkMHpAhUQsaQKHW1QB14Q6AEwD3oECAkQAQ#v=onepage&q=Chelating%20agent%20can%20be%20used%20with%20bio-fertilizers%20application&f=false

Chelated micronutrients are fertilizers where the micronutrient ion (for example Fe or iron) is surrounded by a larger molecule called a ligand or chelator. Ligands can be natural or synthetic chemicals. These compounds combined with a micronutrient forms a chelated micronutrient. Chelators in soil increase the solubility, and thus availability of certain metal micronutrients to plants. For example, in soil with high pH levels, chelating agents will bind insoluble iron, converting it into a water soluble form that is available for plant uptake. Cheated product for adding to fertilizer, the chelating agent is ground to a fine powder. It is then mixed with the sulfate of the desired micronutrient.