Legume Inoculation Cuts Fertilizer Need
Legumes continue to gain importance in Manitoba agriculture due to their ability to fix atmospheric nitrogen. When properly inoculated and well‑nodulated, legumes can meet their nitrogen needs without commercial nitrogen fertilizer.
Legumes form a symbiotic relationship with Rhizobium bacteria. Through a process called nitrogen fixation, rhizobia convert atmospheric nitrogen in soil air spaces into forms the plant can use.
- Atmospheric nitrogen is abundant and continually replenished.
- Effective nitrogen fixation reduces or eliminates the need for nitrogen fertilizer.
Root Nodules
Nitrogen fixation occurs in root nodules, formed when rhizobia infect legume roots.
- Nodule size and shape vary with legume species.
- Rhizobia are host‑specific—only specific bacteria can infect and fix nitrogen in each legume type.
- Sometimes infection occurs without nitrogen fixation; in other cases, bacteria fix nitrogen but cannot infect the root.
Why Inoculation Is Necessary
Inoculation is the process of adding effective Rhizobium strains to the seed before planting.
All legume seed should be inoculated—even if the field has grown that legume before.
This ensures:
- High populations of effective nitrogen-fixing bacteria
- Early, strong nodulation
- Adequate nitrogen supply during early growth
Commercial Rhizobium Inoculants Are Legume‑Specific
Examples include inoculants for:
- Alfalfa group: alfalfa, sweet clover
- Birdsfoot trefoil
- Sainfoin
- Cicer milkvetch
- Pea group: field pea, garden pea, lentils
- Clover group: red, white, alsike clover
- Lupins
- Bean group: dry beans
- Fababeans
- Soybeans
Using the correct strain ensures early and effective nodulation.
Methods of Inoculation
Seed should be sown immediately after inoculation.
1. Slurry Method (Preferred)
Mix the inoculant with water as directed to form a slurry.
Stir onto seed until evenly coated.
Seed should appear dry if done correctly.
Stir onto seed until evenly coated.
Seed should appear dry if done correctly.
2. Adhesive Solution Method
A variation of the slurry method using an adhesive solution.
- Improves adherence of inoculum
- Provides nutrients for rhizobia
- Can be made by mixing 2 tbsp (25 mL) corn syrup into 1 L (1 qt) water
3. Dry Application (Not Recommended)
- Wastes inoculant
- Requires 2–3× more product
- Produces poorer nodulation
4. Pre-Inoculated Seed
- Convenient and effective when properly stored
- Often uses polymer coatings to extend shelf life
- Must be handled carefully to preserve rhizobia viability
Ensuring Successful Nodulation
Handling Inoculants
- Use before expiry date
- Store in a cool, dark place (ideally refrigerated)
- Protect from heat and sunlight
Handling Inoculated Seed
- Seed immediately after treatment
- If delayed, store up to 2 days in cool, dark conditions
- Re‑inoculate if delays exceed 48 hours
Soil Conditions
- Rhizobia do not survive in acid soils; lime acidic soils before planting
- Some acid‑tolerant rhizobia strains exist for alfalfa
- Seed should be placed in moist, but not waterlogged, soil
- Waterlogged conditions reduce soil oxygen and can kill rhizobia
Seed Disinfectants
- Most Manitoba seed treatments are safe
- Fungicide‑treated seed requires 2–3× more inoculant
- Rhizobia cannot tolerate mercury or copper treatments
- In this case, mix inoculum with cracked wheat or sawdust and drill into soil ahead of seed
Checking for Nitrogen Fixation
Dig up a plant and examine nodules:
- Pink or red interior: Active nitrogen fixation
- White: Immature nodules
- Green: Ineffective nodules
Crop Management
Good inoculation practices are essential, but legumes still require:
- Proper seedbed preparation
- Adequate fertility (except nitrogen)
- Good moisture conditions
- Appropriate pest and weed management
Legumes “do not grow on nitrogen alone”—overall management determines crop success.
Summary
- Inoculate all legume seed before planting.
- Use legume‑specific commercial inoculants.
- Store inoculant in a cool, dry place.
- Use the slurry method for best results.
- Seed immediately after inoculation.
- Sow into a moist, well-prepared seedbed.