Phosphorus

Soil Health and Productivity

Important Facts About Soil Phosphorus (P)

  • P Concentration in Plants:

    • Ranges from 0.1% to 0.5%.

    • Considerably lower than Nitrogen (N) and Potassium (K).

  • Plant Absorption Forms:

    • Plants primarily absorb either orthophosphate ions:

    • H<em>2PO</em>4−H<em>2PO</em>4^- (dihydrogen phosphate)

    • HPO42−HPO_4^{2-} (hydrogen phosphate)

    • Absorption depends on soil pH.

  • Organic Phosphorus Compounds:

    • Studies indicate that plants can absorb soluble, low-molecular weight organic P compounds (e.g., nucleic acids, phytin).

    • These organic P compounds are likely converted to H<em>2PO</em>4−H<em>2PO</em>4^- in the rhizosphere.

  • Average Soil Nutrient Ratios:

    • C:N:P:S ratio approximately 120:10:1:1.

  • Functions of Phosphorus in Plants:

    • Energy storage and transfer.

    • Essential for root growth.

  • Types of Orthophosphate:

    • Primary Orthophosphate

    • Secondary Orthophosphate

Phosphorus Cycle Overview

  • Phosphorus Sources and Sinks:

    • Crop P Removal: Plants absorb phosphorus.

    • P Loss Mechanisms: Includes runoff, erosion, and leaching.

    • Fertilizer P Contribution: Uses forms of phosphorus like H<em>2PO</em>4H<em>2PO</em>4 and HPO42−HPO_4^{2-}.

    • Soil P Reservoirs: Include adsorbed P, primary and secondary minerals.

  • Key Phosphorus Processes:

    • Desorption and Adsorption: Processes of phosphorus entering and binding to soil particles.

    • Dissolution and Precipitation: Movement of phosphorus between solid and solution phase.

    • Plant Uptake: Direct absorption of P by plant roots.

    • Microbial Activity: Involvement of fungi, bacteria, and protozoa in P cycling.

Soil pH and Nutrient Availability

  • Nutrient Availability Chart by Soil pH:

    • Shows how soil pH levels influence the availability of various essential nutrients (Iron, Manganese, Boron, Copper, Zinc, Nitrogen, Phosphorus, Potassium, Sulphur, Calcium, Magnesium, Molybdenum).

  • pH Ranges and Nutrient Chemistry:

    • Highly acidic (pH < 7.2): Greater concentration of H<em>2PO</em>4−H<em>2PO</em>4^- compared to HPO42−HPO_4^{2-}.

    • Neutral (pH 7.2): H<em>2PO</em>4−H<em>2PO</em>4^- = HPO42−HPO_4^{2-}.

    • Alkaline (pH > 7.2): HPO<em>42−HPO<em>4^{2-} predominates over H</em>2PO4−H</em>2PO_4^-.

  • Reactivity of Orthophosphates:

    • Acidic conditions result in reactions with Aluminum (Al-phosphate) and Iron (Fe-phosphate) resulting in insoluble compounds.

    • Alkaline conditions lead to the formation of Calcium phosphates (e.g., mono, di, tri calcium phosphate).

VAM Biofertilizer and Phosphorus Absorption

  • VAM Overview:

    • VAM stands for Vesicular Arbuscular Mycorrhiza.

    • Acts as a biofertilizer, a type of fungi, critical for plant nutrient uptake, particularly phosphorus.

    • Type: Endomycorrhizae.

    • Functions:

    • Release organic acids to solubilize phosphate compounds.

    • Aid the translocation of P into plant systems.

    • Engage in an obligate symbiotic relationship where fungi receive carbon from the plant.

Fungal Associations for Phosphorus Uptake

  • Components of Mycorrhizal Structures:

    • Chlamydospore, External Feeding, Hyphae, Arbuscule (internal hyphae), Vesicle, and root hair.

    • Anatomy of fungal colonization includes Xylem, Epidermis, Cortex, and Hartig net.

    • VAM colonized roots create a P depletion zone around them, enhancing P uptake.

Phosphorus Mineralization and Immobilization in Soils

  • Mineralization Process:

    • Conversion of organic P into inorganic forms, primarily H<em>2PO</em>4−H<em>2PO</em>4^- and HPO42−HPO_4^{2-}.

  • C:P Ratio Influence:

    • Affects net mineralization or immobilization of P based on varying C:P ratios:

    • Ratios < 200: Net mineralization of organic P.

    • Ratios 200-300: No net gain or loss of inorganic P.

    • Ratios > 300: Net immobilization of inorganic P.

  • Phosphatase Reaction:

    • Reaction of organic P with water:

    • R−O−P−O−R-O-P-O^- + H2O<br>ightarrowH−O−P−O−H_2O <br>ightarrow H-O-P-O^- + R−OHR-OH.

Factors Influencing Phosphorus Fixation in Soils

  • Soil Minerals:

    • Adsorption/desorption reactions significantly influenced by iron and aluminum oxides.

    • The type of clay affects the Cation Exchange Capacity (CEC): High clay content increases the Capacity.

    • Example: Kaolinite (lower CEC) vs. Montmorillonite (higher CEC).

  • Soil pH Impact:

    • Altered phosphorus fixation based on soil pH levels which dictate mineral reactivity.

  • Cation and Anion Effects:

    • Divalent cations (e.g., Ca2+^{2+}) adsorb more P than monovalent (e.g., Na+^+), especially at lower pH levels.

  • Soil Organic Matter (OM):

    • Increases P availability via:

    • Soluble organophosphate compounds.

    • Organic anions replacing H<em>2PO</em>4−H<em>2PO</em>4^-.

    • Coating of Fe/Al oxides by humus.

  • Additional Factors:

    • Time and temperature impact rate of P adsorption; warmer regions exhibit higher adsorption rates.

    • Flooding events can temporarily increase available P due to the conversion of Fe3+^{3+} - P to a more soluble form (Fe2+^{2+} - P).

Visual Deficiency Symptoms of Phosphorus

  • Overall stunted growth of plants.

  • Affects older leaves, leading to a darker green coloration.

  • Presence of bright red stems, bluish-green metallic luster, and purple leaf color as indicators of P deficiency.