• Phosphorus is taken up by the plant in the form of H₂PO₄^-, HPO₄^-2, PO₄^-3 through diffusion and mass flow action.
• The P availability mainly depends on pH. In acid soils (Al & Fe) the presence of Al, Fe, Mn, P gets fixed as AlPO₄, FePO₄ and not available to the plants. Some times as CaPO₄. These are insoluble in H₂O.
• Under hilly areas (or) high rainfall areas, all the cations will be leached leaving Fe, Al and Mn. The P availability will be reduced.
• Ideal pH for available P => 6.5 - 7.5
• If pH > 8.5 the fixation will be more & < 6.5 the fixation will be more.

Forms of P

• Organic P: Nucleic acid and Phospho lipid
• Rock Phosphate - acid soluble. If the organic matter content is high the availability of P is more since it is soluble in acid. It is highly suited to plantation crops. Rock Phosphates is black in colour. Roots also exudates acids, which will solublises the P.
• Fixation is high so the P₂O₅ deficiency is 15 – 35%.

Phosphorus Mineralization

• C : N : P = 100 : 10 : 1 and if C: P ratio is more than 100 : 1 => immobilization of ‘P’ occurs.

Soil P its origin and Nature

• Soil P exists in many primary and secondary compounds.
• The apatite group of primary minerals is the original source i.e. 55% of soil P.

Fixation of phosphorus in soil ^{UPPSC 2021}

• Phosphorus occurs in most plants in concentrations between 0.1 and 0.4%.
• Plants absorb either H₂PO₄^- or HPO₄^2- Ortho PO₄^3- ions
• Absorption of H₂PO₄^- is greatest at low pH values, whereas uptake of HPO₄^2- is greater at higher values of soil pH, plant uptake of HPO₄^2- is much slower than H₂PO₄^-

Available form of P in acid soil

H₂PO₄^- In less alkali or neutral: HPO₄^2- and when alkalinity is more i.e. high pH: PO₄^3-.

👉🏻 The factors affecting P fixation are:

1. Clay minerals

• The PO₄ is fixed by clay minerals by reacting with soluble aluminum which originates from the exchange sites or from lattice dissociation to from a highly insoluble AlPO₄.

2. Iron and Aluminum (fixation in acid soils)

• The formation of Iron and aluminum PO₄ in the soil results from the combination of P with these metals in solution and their oxides hydroxides in acid soil.
• 2Al + 3 H₂PO₄ → Al₂ (PO₄)₃ + 2 H₂O + 2H+

3. Exchange cations and calcium carbonate (Fixation alkaline soils)

• In calcareous soils, free CaCO₃ is a potent source for ‘P’ fixation. P fixation in calcareous soil involved a rapid monolayer sorption of P in dilute concentration. In CaCO₃ surfaces and form less soluble compounds of di and tricalcium PO₄.
• Ca(H₂PO₄)₂ + 2CaCO₃ → Ca₃(PO₄)₂ + 2CO₂ + 2 H₂O

4. Organic matter

• Organic PO₄ can be fixed by soil organic matter also influences in Organic PO₄ fixation.
• The acids produced during the transformations of Organic matter could decreases the pH and increases fixation by the solubilization of Fe and Al.

Functions

• Due to deficiency of single element phosphorus, plants cannot complete their life cycle hence ‘P’ is called key to life.
• Nitrogen governs the above earth growth whereas ‘P’ governs the root growth i.e. below earth growth.
• It has a greater role in energy storage and transfer.
• It is a constituent of nucleic acid, phytin and phospholipids.
• It is essential for cell division and development. (Meristem Region)
• P compounds act as energy currency within plants. The most common P energy currency is that found in ADP and ATP. Transfer of the energy rich PO4 molecules from ATP to energy requiring substances in the plant is known as “Phosphorylation”
• It stimulates early root development and growth and there by helps to establish seedlings quickly.
• It gives rapid and vigorous start to plants strengthens straw and decreases lodging tendency.
• It is essential for seed formation because larger quantities of P is found in seeds and fruits. Phytic acid is the principle storage from of phosphorus in seeds.
• Counteracts the excess N. Increases grain to straw ratio.
• Increases Rhizobia activity, increases the formation of root nodules thus helping in more N - fixation.

Deficiency of P

👉🏻 P is mobile in plants and when a deficiency occurs it is translocated from older tissues to the active meristematic regions.

• The marked effect of P deficiency is retarding overall growth. [Late Maturity]
• Leaves will show characteristic bluish green colour.

• It arrests metabolism resulting in reduction of total N of Plants.
• Reduced sugar content. Poor seed and fruit development.
• Premature leaf fall. Restrict root growth.
• Develops necrotic area on the leaf petiole and in the fruit.

Toxicity of phosphorus

• Profuse root growth i.e. lateral and fibrous root lets. [Early maturity and less overall growth]
• It develops normal growth having green leaf colour.
• It may cause in some cases trace elements deficiencies i.e. Zinc and Iron.

Explore More 🔭

🟢 Strengths and weaknesses of acids and bases

- Tisdale, S.L., Nelson, W.L., Beaton, J.D. Havlin, J.L.1997. Soil fertility and Fertilizers. Fifth edition, Prentice hall of India Pvt. Ltd, New Delhi
- Singh, S.S.1995. Soil fertility and Nutrient Management. Kalyani Publishers, Ludhiana
- Maliwal, G.L. and Somani, L.L. 2011. Soil Technology. Agrotech

• Phosphorus is taken up by the plant in the form of H₂PO₄^-, HPO₄^-2, PO₄^-3 through diffusion and mass flow action.
• The P availability mainly depends on pH. In acid soils (Al & Fe) the presence of Al, Fe, Mn, P gets fixed as AlPO₄, FePO₄ and not available to the plants. Some times as CaPO₄. These are insoluble in H₂O.
• Under hilly areas (or) high rainfall areas, all the cations will be leached leaving Fe, Al and Mn. The P availability will be reduced.
• Ideal pH for available P => 6.5 - 7.5
• If pH > 8.5 the fixation will be more & < 6.5 the fixation will be more.

Forms of P

• Organic P: Nucleic acid and Phospho lipid
• Rock Phosphate - acid soluble. If the organic matter content is high the availability of P is more since it is soluble in acid. It is highly suited to plantation …

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