🐛 Phosphorus Biofertilizers

This lesson builds core elective concepts in BSc Agriculture with practical applications and exam-oriented clarity.

---

Phosphorus Biofertilizers

Phosphorus (P) is the second most important macronutrient after nitrogen, essential for energy transfer (ATP), nucleic acid synthesis, and root development. Although Indian soils contain considerable total phosphorus, only 0.1% exists in plant-available form. Most phosphorus is locked as insoluble calcium phosphates (alkaline soils), iron and aluminium phosphates (acidic soils), or organic phosphorus.

Phosphate Solubilizing Bacteria (PSB)

PSB are microorganisms capable of converting insoluble phosphorus compounds into soluble, plant-available forms (H2PO4- and HPO4^2-).

Important PSB Organisms

Organism	Characteristics
Bacillus megaterium var. phosphaticum	Most widely used PSB in India; effective in alkaline soils
Bacillus polymyxa	Produces organic acids and phosphatases
Pseudomonas striata	Effective in both acid and neutral soils
Pseudomonas fluorescens	Dual role — P solubilization + biocontrol
Aspergillus awamori	Fungal PSB; high organic acid producer
Penicillium bilaii	Commercial product "JumpStart" for wheat and canola

Mechanism of Phosphorus Solubilization

PSB solubilize inorganic phosphorus primarily through:

1. Organic acid production — gluconic acid, citric acid, oxalic acid, and succinic acid chelate Ca2+, Fe3+, and Al3+ ions, releasing bound phosphate
2. H+ extrusion — proton release via respiratory H+ or NH4+ assimilation lowers rhizosphere pH
3. Phosphatase enzymes — acid phosphatase and alkaline phosphatase mineralize organic phosphorus compounds (phytate, phospholipids, nucleic acids)
4. Siderophore production — chelates iron, indirectly releasing Fe-bound phosphate

Benefits of PSB Application

• Solubilizes 15-25 kg P2O5/ha from fixed soil phosphorus
• Can replace 25-30% of recommended phosphatic fertilizer application
• Improves phosphorus use efficiency of applied DAP/SSP
• Produces growth-promoting substances (IAA, siderophores)
• Recommended dose: 200 g/acre as seed treatment or 2-3 kg/acre for soil application

Vesicular Arbuscular Mycorrhiza (VAM/AM Fungi)

Arbuscular Mycorrhizal (AM) fungi (formerly VAM) belong to the phylum Glomeromycota and form symbiotic associations with roots of over 80% of terrestrial plant species. They are obligate symbionts and cannot be cultured on artificial media.

Important AM Genera

• Glomus (most common — G. mosseae, G. intraradices, G. fasciculatum)
• Gigaspora
• Acaulospora
• Scutellospora

Mechanism of Phosphorus Mobilization

1. AM fungal hyphae penetrate root cortical cells, forming arbuscules (tree-shaped structures) that serve as the interface for nutrient exchange
2. External hyphae extend far beyond the root depletion zone (up to 10-15 cm), effectively increasing the root absorption surface by 100-1000 times
3. Hyphae absorb phosphate ions from soil solution and transport them as polyphosphate granules through the hyphal network to arbuscules
4. Phosphorus is released at the arbuscular membrane in exchange for carbon (sugars) from the plant (the plant provides 10-20% of its photosynthate to the fungus)

Additional Benefits of AM Fungi

• Enhance uptake of zinc, copper, and micronutrients
• Improve drought tolerance through better water absorption and osmotic adjustment
• Increase disease resistance by competing with root pathogens and inducing defense responses
• Improve soil aggregation through glomalin production (a glycoprotein that binds soil particles)
• Particularly beneficial in phosphorus-deficient soils and for crops like citrus, papaya, onion, maize, and plantation crops

Application

AM fungi are applied as soil inoculant (50-100 g/plant for tree crops) or mixed with nursery media. Since they are obligate symbionts, mass production requires trap cultures using host plants like sorghum, maize, or onion grown in sterilized soil-sand media.

---

Summary Cheat Sheet

Topic	Key takeaway
Main focus	Phosphorus Biofertilizers — Phosphate Solubilizing Bacteria (PSB), VAM/AM fungi, mechanisms, and agricultural applications.
Section context	Revise this lesson with the rest of Biopesticides and Biofertilizers for stronger conceptual continuity.