Phosphorus Cycle or Transformation
Phosphorus is only second to nitrogen as a mineral nutrient required for plants, animals and microorganisms. It is a major constituent of nucleic acids in all living systems essential in the accumulation and release of energy during cellular metabolism. This element is added to the soil in the form of chemical fertilizers, or in the form of organic phosphates present in plant and animal residues. In cultivated soils it is present in abundance (i.e. 1100 kg/ha), but most of which is not available to plants, only 15 % of total soil phosphorus is in available form. Both inorganic and organic phosphates exist in soil and occupy a critical position both in plant growth and in the biology of soil.
Microorganisms are known to bring a number of transformations of phosphorus, these include:
(i) Altering the solubility of inorganic compounds of phosphorus,
(ii) Mineralization of organic phosphate compounds into inorganic phosphates,
(iii) Conversion of inorganic, available anion into cell components i.e. an immobilization process and
(iv) Oxidation or reduction of inorganic phosphorus compounds. Of these mineralization and immobilization are the most important reactions / processes in phosphorus cycle.
Insoluble inorganic compounds of phosphorus are unavailable to plants, but many microorganisms can bring the phosphate into solution. Soil phosphates are rendered available either by plant roots or by soil microorganisms through secretion of organic acids (eg. lactic, acetic, formic, fumaric, succinic acids etc). Thus, phosphate-dissolving / solubilizing soil microorganisms (eg. species of Pseudomonas, Bacillus, Micrococcus, Mycobacterium, Flavobacterium, Penicillium, Aspergillus, Fusarium etc.) plays important role in correcting phosphorus deficiency of crop plants. They may also release soluble inorganic phosphate (H2PO4), into soil through decomposition of phosphate-rich organic compounds.
Solubilization of phosphate by plant roots and soil microorganisms is substantially influenced by various soil factors, such as PH, moisture and aeration.
In neutral or alkaline soils solubilization of phosphate is more as compared to acidic soils. Many phosphates solubilizing microorganisms are found in close proximity of root surfaces and may appreciably enhance phosphate assimilation by higher plants.
By their action, fungi bacteria and actinomycetes make available the organically bound phosphorus in soil and organic matter and the process is known as mineralization. On the other hand, certain microorganisms especially bacteria assimilate soluble phosphate and use for cell synthesis and on the death of bacteria, the phosphate is made available to plants. A fraction of phosphate is also lost in soil due to leaching. One of the ways to correct deficiency of phosphorus in plants is to inoculate seed or soil with commercial preparations (eg. Phosphobacterin) containing phosphate – solubilizing microorganisms along with phosphatic fertilizers.
Mineralization of phosphate is generally rapid and more in virgin soils than cultivated land. Mineralization is favored by high temperatures (thermophilic range) and more in acidic to neutral soils with high organic phosphorus content. The enzyme involved in mineralization (cleavage) of phosphate from organic phosphorus compound is collectively called as “Phospatases".
The commercially used species of phosphate solubilizing bacteria and fungi are: Bacillus polymyxa, Bacillus megatherium. Pseudomonas strita, Aspergillus, Penicllium avamori and Mycorrhiza