Phosphorus

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Sulphur Phosphorus and Trace Element Nutrition Sulphur Phosphorus Solubilization of Tricalcium Phosphate or Calcium Phytate by Isolates of Soil Fungi Agronomical Aspects

Phosphorus

Phosphorus

Both inorganic and organic phosphates occur in soil. The inorganic forms are compounds of Ca, Fe, AI, and P. The bulk of phosphorus on earth exists as apatites, with the basic formula M10 (PO₄)₆ X,₂. The M refers to calcium and the X is the anion fluorine but it can also be CI- OH- or CO₃ which means that phosphorus can exist as flour, chloro, hydroxy and carbonate apatites. Different substitutions and combinations of M and X may result in 200 different types of phosphorus occurring in nature.

Rock phosphates high in carbonate apatite are commonly mined for fertilizer. The organic phosphorus containing compounds are derived from plants and microorganisms and are composed of nucleic acids, phospholipids and phytin. Organic matter derived from dead and decaying plant debris is rich in organic sources of phosphorus.

The deficiency of phosphorus may occur in crop plants growing in soils containing adequate phosphates. This may be partly due to the fact that plants are able to absorb phosphorus only in an available form. Soil phosphates are -rendered available either by plant roots or by soil microorganisms through secretion of organic acids.

The sulphur Cycle

The sulphur cycle

Therefore, phosphate-dissolving soil microorganisms play some part in correcting phosphorus deficiency of crop plants. They may also release soluble inorganic phosphate (H₂PO₄) into soil through decomposition of phosphate-rich organic compounds. On the other hand, certain microorganisms, through assimilation, may immobilize available phosphates in their cellular material. Such immobilization processes in soil may also contribute to phosphorus deficiency of crop plants.

Sulubilization of phosphates by plant roots and microorganisms is dependent on soil pH. In neutral or alkaline soils having a high content of calcium, precipitation of calcium phosphates takes place. Microorganisms and plant roots readily dissolve such phosphates and render them easily available to plants.

On the contrary, acid soils are generally poor in calcium ions and, therefore, phosphates are precipitated in the form of ferric or aluminium compounds which are not so easily amenable to solubilization by plant roots or by soil microorganisms. If such conditions prevail in acid soils, deficiency of phosphorus in plants may also occur. One of the ways to correct deficiency of phosphorus in plants is to inoculate seed or soil with phosphate-dissolving microorganisms along with phospatic fertilizers.

The phosphorus cycle

Thr phosphorus Cycle

Many fungi and bacteria (for example, Aspergillus, Penicillium, Bacillus and Pseudomonas) are potential solubilizers of bound phosphates as revealed by experiments in pure culture A. Although bacteria have been used in the commercial preparation of phosphate-dissolving cultures to improve the growth of plants, fungi seem to be better agents in the dissolution of phosphates. Phosphate-dissolving bacteria are known to reduce the pH of the substrate by secretion of a number of organic acids such as formic, acetic, propionic, lactic, glycolic, fumaric and succinic acids. Some of these acids (hydroxy acids) may form chelates with cations such as Ca and Fe and such chelation results in effective solubilization of phosphates. Although organic acid production is invariably associated with phosphate solubilization in pure cultures of microorganisms, many workers have not been able to correlate changes in the pH of the medium after growth with the amount of phosphates solubilized.

Labelled phosphate (³²p) has been used to test the phosphate solubilizing property of soil microorganisms and also to find out the extent of phosphorus uptake by the plant. Such experiments have been done in sterilized and unsterilized soils using tricalcium and rock phosphates, apatite and bone meal. The results seem to be inconclusive since several contradictory reports have been made on this aspect.


(A) Microbial solubilization of phospahte
(B) Gallionella minor, (C) G. major

(D) Gallionella minor ferruginea

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Effect of Phosphate Solubilizing Microorganisms on Various Crops in Some Field Experiments in India Trace Elements Manganese Copper Iron Bioaccumulation and Mineral Leaching

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