Rhizobium

Introduction

Rhizobium is one of the most recognized and widely used nitrogen-fixing bacteria in legume-based farming systems. Its ability to form symbiotic nodules on legume roots makes it a foundational component of sustainable agriculture. Rhizobium helps reduce the need for chemical nitrogen fertilizers, making it particularly valuable in low-input and organic farming systems.

Scientific Description

Rhizobium spp. are gram-negative, rod-shaped, motile bacteria within the Alphaproteobacteria class. They are best known for their symbiotic nitrogen fixation with leguminous plants, forming root nodules where atmospheric nitrogen (N₂) is converted into ammonia (NH₃), which the plant can assimilate.

Key characteristics:

• Symbiosis: Forms highly specific associations with host legumes (e.g., Rhizobium leguminosarum with peas, R. phaseoli with beans).
• Nitrogenase activity: Operates under anaerobic conditions inside nodules; oxygen is regulated by leghemoglobin.
• Soil survival: Exists as free-living cells between crop cycles but survives best in rhizosphere proximity or in moist carriers.

Benefits

• High-Efficiency Nitrogen Fixation: Supplies up to 150–200 kg N/ha/year in legume crops, reducing the need for chemical N fertilizers.
• Crop-Specific Targeting: Effective only with compatible legumes, ensuring focused nutrient supply.
• Supports Soil Fertility: Adds organic nitrogen to the soil, benefiting subsequent crops in rotation.
• Eco-Friendly: Reduces greenhouse gas emissions by lowering synthetic fertilizer use.
• Low Cost: Inoculation is cost-effective and easy to apply for farmers.

Uses

• Legume Crops: Pea, chickpea, lentil, groundnut, soybean, cowpea, mung bean, pigeonpea, and French bean.
• Application Methods:
  • Seed inoculation: Most common method (5 g/kg seed with sticker).
  • Soil drenching: For transplanted legumes.
  • Co-inoculation: Often combined with phosphate solubilizers for balanced nutrition.
• Crop rotation systems: Residual nitrogen can benefit non-legume crops grown in the following season.

Trivia

• The Rhizobium-legume symbiosis is one of the most studied plant-microbe interactions in biology.
• Leghemoglobin inside nodules gives them a pink color and helps maintain oxygen balance.
• Rhizobium-based inoculants are part of government-promoted biofertilizer schemes in India and Africa.
• Misapplication on non-host crops or wrong strains leads to zero benefit—strain matching is critical.