Azotobacter

Introduction

Azotobacter is a genus of free-living nitrogen-fixing bacteria that plays a vital role in improving soil nitrogen levels, especially in non-leguminous cropping systems. It fixes atmospheric nitrogen independently in the soil, enhancing fertility and promoting plant growth without the need for synthetic nitrogen inputs. It is particularly useful in cereals, vegetables, and horticultural crops.

Scientific Description

Azotobacter spp. are gram-negative, aerobic, motile, free-living bacteria commonly found in neutral to alkaline soils. They belong to the family Pseudomonadaceae, with Azotobacter chroococcum being the most widely used species in agriculture.

Biological traits:

• Biological Nitrogen Fixation (BNF): Converts atmospheric N₂ into ammonia using nitrogenase enzyme under aerobic conditions.
• Growth-Promoting Substances: Produces indole acetic acid (IAA), gibberellins, cytokinins, and vitamins (notably B12).
• Siderophore Production: Chelates iron and improves micronutrient availability.
• Phosphate Solubilization: Some strains can mobilize fixed phosphorus and zinc.
• Cyst Formation: Forms thick-walled cysts for survival under drought and nutrient stress.

Benefits

• Nitrogen Fixation in Non-Legumes: Supplies 20–40 kg N/ha/year, reducing chemical fertilizer needs.
• Supports Root Development: Hormone production leads to better root architecture and seedling vigor.
• Improves Germination and Yield: Enhances seed viability and biomass accumulation.
• Stress Resistance: Helps plants cope with drought and salinity through antioxidant enzymes and osmolyte accumulation.
• Soil Health Improvement: Promotes microbial diversity and nutrient cycling.

Uses

• Crops Benefited: Cereals (rice, wheat, maize), vegetables, sugarcane, cotton, banana, tubers, and even plantation crops.
• Application Methods:
  • Seed treatment: 5–10 g/kg seed (powder) or 5 mL/kg (liquid)
  • Soil application: 2–4 kg/acre mixed with compost or FYM
  • Seedling root dip: For banana, tomato, chili, brinjal, and others
  • Drip fertigation: For horticulture and plantation crops
• Ideal Conditions: Neutral to alkaline soils with moderate organic content

Trivia

• Azotobacter was one of the first free-living nitrogen fixers ever discovered (Winogradsky, 1901).
• Unlike Rhizobium, it does not require a plant host to function.
• Some strains are used in biofertilizer cocktails with Phosphobacteria and Trichoderma.
• Often recommended in organic certification programs for vegetables and sugarcane.