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Soybean – Inoculation

Purpose of Inoculation

One of the most important inputs for any pulse crop is inoculant. Soybeans are not native to the Canadian Prairies and so crop must be inoculated with soybean inoculant that contains rhizobia (Bradyrhizobium japonicum) bacteria. Soybeans are a legume crop and are capable of fixing 50% to 60% or more of their required nitrogen. While they need more nitrogen to maximize yield than what is provided by fixation, it is not practical to apply additional nitrogen as fertilizer, as it can be antagonistic in the development of the fixation relationship and seldom results in increased yields. 

Inoculant Formulations

Soybean inoculants come in three formulations.  All inoculant formulations will perform equally well if the inoculant is properly applied and if environmental conditions are ideal. Under adverse conditions the best performing formulation should be granular, followed by peat, and then liquid.

TIMING

  • Generally, inoculate your seed the day you’re seeding, but different brands or types have different storage limits and recommended application timing.
  • Some types of inoculants can also be mixed with fertilizer or pesticides. When choosing the right soybean inoculant, talk to your input supplier and read all labels carefully.

DOUBLE INOCULATION
  • In fields where soybeans do not have a history of being grown, double inoculation is recommended.
  • Usually double inoculation is in the form of a seed-applied liquid plus an in-furrow granular inoculant.
  • Check with your seed supplier as some companies supply seed with a liquid inoculant already on the seed so then only a granular inoculant is required at seeding.
  • If using seed treatments, the compatibility of seed treatments and inoculants should be confirmed, especially if applying liquid inoculants on seed.

SINGLE INOCULATION
  • Single inoculation is the use of one inoculant formulation or placement technique. Once introduced to the soil, rhizobia can survive in the soil for many years. Therefore, the amount of applied inoculant can be reduced once soybeans have become established in a crop rotation. If considering single inoculation, follow the checklist below:
    • Field has had at least two previous soybean crops.
    • Previous soybean crops have nodulated well.
    • Most recent soybean crop within the past four years.
    • No significant flooding or drought.
    • All four above criteria have been met.

Peat Powder Inoculant
  • Applied directly to the seed with a non-toxic sticking agent, this formulation is a finely ground peat that contains over a billion rhizobia per gram.
  • Powder/peat formulations are more durable and less prone to desiccation and seed treatment damage, compared to liquid formulations. The bacteria can still be killed by desiccation so the same precautions should be taken as with liquid.
  • Some peat-based powder inoculants require the use of a sticker. Adhesion to the seed can be enhanced if the seed is slightly damp during inoculation.
  • Peat powder inoculant is one of the most common types used in Canada.

Liquid Inoculant
  • This formulation, which also contains over a billion rhizobia per gram, is applied directly to the seed, and because it comes in liquid form, a sticking agent is typically included in the fluid.
  • Liquid inoculant comes in bags that make it easy to distribute evenly onto the seed while it is being augured into a truck box or through a drill fill.
  • Liquid-based products offer convenience and better control of application rate compared to other forms. However, they are also more susceptible to damage prior to seeding from environmental extremes and seed treatments than other inoculant forms.
  • If treated seed is planted immediately into a moist seedbed, liquid formulations perform well.

Granular Soil Inoculant
  • Unlike peat powder or liquid inoculants, granular soil inoculant is not applied directly to the seed but, rather, with the seed in the seed row.
  • This formulation does, however, contain the same amount of rhizobia as both the powder and liquid inoculants and is gaining in popularity because of its convenience.
  • Under cold or very dry spring seedbed conditions, granular soil inoculation has shown considerable potential for producing large, stable yields and minimizing the risk of growing the crop.
  • Granular formulations are the least prone to exposure damage. They allow for precise application rates when an additional seed cart compartment is available.
  • Granular inoculants are less affected by seed-applied fungicides than other forms of inoculants.

Maximizing Rhibozia Survivability
  • Once the proper inoculant is chosen, steps should be taken to ensure maximum rhizobia survivability.
  • Rhizobium bacteria (either on the seed or in the package) die if they are exposed to stress such as high temperature, drying winds, or direct sunlight.
  • Inoculant must be stored in a cool dark place prior to use and must be used before the expiry date. Following application of the inoculant, plant into moist soil as soon as possible.
  • Inoculants are sensitive to granular fertilizer. Banding fertilizer to the side and/or below the seed is recommended. Never mix inoculant with granular fertilizer.
  • Inoculants are also sensitive to some seed-applied fungicides. Check the label of both the inoculant and seed treatment for compatibility.
  • When using a combination of fungicide and inoculant, apply the fungicide to the seed first, allow it to dry, and apply the inoculant immediately prior to seeding.

How do I know if the inoculant was successful?

TIMING
  • The effectiveness of inoculation can be checked by examining the pulse crop at early flowering. It may take three to four weeks after seed germination before nodulation reaches a point where it can be evaluated.

HOW TO CHECK
  • Soybean are less effective in their ability to fix nitrogen, compared to faba bean, peas, chickpeas and lentils. They are better than dry bean.
  • The nodules can be easily seen when a plant is pulled from the ground. The best way to check for nodulation is to dig a plant and gently remove the soil from the roots by washing in a bucket of water.
  • Nodules are fragile and readily pull off if the roots are pulled out of the soil.
  • Watch: Nodulation Scoring Video 

WHAT SHOULD I EXPECT?
  • Nitrogen fixation is synchronized with plant growth, supplying the crop nitrogen during rapid vegetative growth.
  • Seed applied inoculant should result in nodules forming on the primary root near the crown.
  • If the inoculant was soil applied (granular), nodules should be found on primary and secondary roots.
  • If nitrogen fixation is active, the nodules will be pink or red on the inside.
  • Lack of nodules indicates rhizobia did not infect the pulse plant.
  • Lack of a pink colour (usually green or cream coloured) indicates the rhizobia are not fixing nitrogen.
  • Nitrogen fixation declines once plants begin pod formation and seed development.

NATURALLY OCCURRING
  • Some sites show little to no response to inoculation with any formulation as these sites had previous pulse crops or naturally occurring (native) rhizobia.

USE AT RECOMMENDED RATES
  • When seeding on land with no known history of pulses, it’s best to use inoculant at recommended rates – although some rhizobia occur naturally in soils, it’s important to ensure sufficient numbers of the correct strain of highly effective rhizobia are present when the seed germinates.

WATCH – Nodulation Scoring Video

Resources

Special thanks to Saskatchewan Pulse Growers and Manitoba Pulse & Soybean Growers.