Introduction
Azolla is a genus of small, aquatic, free-floating pteridophytes (water ferns) belonging to the family Salviniaceae. It is widely recognized as a potent bio-fertilizer in wetland rice cultivation owing to its obligate symbiotic association with the filamentous, heterocystous cyanobacterium Anabaena azollae, which facilitates biological nitrogen fixation (BNF).
Taxonomy, Diversity and Symbiotic Association
Represented globally by species such as Azolla pinnata (predominant in India), A. microphylla, and A. caroliniana, this miniature fern harbors the cyanobiont Anabaena azollae within the mucilage-filled cavities of its dorsal leaf lobes. Within these specialized micro-environments, cyanobacterial heterocysts provide an anaerobic niche that shields oxygen-sensitive nitrogenase enzymes, allowing atmospheric nitrogen fixation:
N₂ + 8H⁺ + 8e⁻ + 16MgATP → 2NH₃ + H₂ + 16MgADP + 16Pᵢ
Mass Cultivation and Agronomic Application Methods
- Pit/Silpauline Method for Mass Production: Cultivated in low-cost artificial pits (2 m × 2 m × 0.2 m) lined with UV-stabilized silpauline sheets. Pits are filled with 10–15 kg of fertile soil, 2 kg of decomposed cow dung slurry, and 30 g of Single Super Phosphate (SSP) in 10 cm standing water. Inoculated with 1 kg mother culture, biomass doubles every 3 to 5 days.
- Green Manuring: Grown as a monocrop in inundated fields for 20–30 days prior to paddy transplantation, then incorporated into the soil using a rotary tiller or conoweeder. This incorporates approximately 30–40 kg N/ha of organic nitrogen.
- Dual Cropping in Rice: Inoculated at 1.5–2.0 t/ha fresh biomass 7–10 days after transplanting rice. It spreads over the water surface, forming a protective mat that gradually decomposes and releases nutrients as the rice canopy shades it out.
Multi-Dimensional Agronomic Benefits
- Nutrient Enrichment: Mineralization of decomposed Azolla biomass releases organic nitrogen alongside macro-elements (C, H, O) and trace micronutrients such as Zinc (Zn) and Iron (Fe).
- Physical Weed Suppression: Rapid formation of an interlocking vegetative blanket blocks light penetration to the benthic layer, physically inhibiting weed seed germination and growth.
- Soil and Water Conditioning: Reduces evaporative water loss from the floodwater surface, buffers water temperature, increases Soil Organic Carbon (SOC), and improves bulk density and soil aeration.
Technical Limitations and Constraints
- Thermal Sensitivity: Azolla prefers temperatures between 20°C and 30°C; ambient temperatures exceeding 38°C induce rapid chlorosis, fragmentation, and vegetative mortality.
- Hydrological Constraints: Strict requirement for continuous standing water (5–10 cm) restricts its deployment entirely to lowland submerged paddy, rendering it unviable for upland or aerobic rice systems.
- Phosphorus Dependency: Low available phosphorus in soils drastically limits vegetative multiplication and nitrogenase activity, requiring supplementary phosphatic fertilization.
- Pest Infestation: Highly vulnerable to aquatic insects, webworms, and snail predation, which can decimate entire floating mats within days.
Conclusion
Azolla represents a cost-effective, ecological alternative to synthetic chemical fertilizers, contributing directly to integrated nutrient management. By integrating Azolla bio-fertilization under national schemes such as PM-PRANAM and Paramparagat Krishi Vikas Yojana (PKVY), farmers can substantially reduce urea consumption while enhancing the long-term biological health of wetland agroecosystems.