Rhizobium bacteria have a symbiotic relationship with legumes to convert atmospheric nitrogen (N₂) to a plant-available form. This process is called nitrogen fixation.

Sufficient numbers of effective rhizobium bacteria must be present to ensure that plants are well nodulated and able to meet the N needs of the crop. Since many soils do not contain sufficient numbers of Rhizobium bacteria, inoculation is recommended to assure early formation of functioning nodules.

The most common forms of inoculant formulations are: granular, powdered peat, liquid and frozen concentrates and pre-inoculated seed. All but granular inoculants are applied by coating the seed with a prepared culture of the required strain of Rhizobium bacteria. Granular inoculants are designed for application in the furrow with the seed. Compared with peat-based and liquid inoculant, the granular form is more convenient to use and seems to be more effective in dry soils. However, granular inoculants are more expensive and may require special modifications to seeding equipment to ensure placement in the seed zone.

Pre-inoculation of seed has proven effective for nodulation of alfalfa and other forage legumes and for current season use on soybeans.

Each legume or group of legumes requires a unique species of Rhizobium to form nodules and fix N. Commercial inoculants are prepared for specific groups of legumes as follows:

• alfalfa group – for alfalfa and sweet clover
• birdsfoot trefoil – for birdsfoot trefoil
• clover group – for red, white and alsike clover
• fababean group – for fababeans including broad and horse beans
• field bean group – for field, garden, navy, pinto and other coloured beans
• pea and lentil group – for field, garden, flat peas and lentils
• soybean group – for soybeans only

Labels will contain information on proper storage, handling and application of inoculant. Improper storage, which allows drying or heating, will reduce bacteria viability.

Most legumes are very efficient and derive almost all their N needs through N fixation, so no additional N fertilizer is required. However, N fixation may be reduced by acidic soil conditions, toxic seed treatments, desiccation in dry seedbeds, high soil nitrate levels or fertilizer applications.

Dry beans are rather inefficient at fixing N and obtain less than half of their requirements through fixation.

Recent field studies indicate that treatment with Rhizobium inoculant is ineffective on current dry bean cultivars grown in Manitoba. Bean response to applied N has been large and recommendations based on soil N and expected yield are found in Appendix Table 12. The field bean production system influences the response to applied N. When beans are grown in wide rows and inter-row cultivation is used to control weeds and for hilling, some mineralization of organic N occurs due to soil disturbance. When beans are grown in narrow rows, applied N modifies the plant architecture to make it more suitable for direct combining. Plants are taller and pods are held higher off the ground which increases the harvestable yield.

High N rates on dry beans have a minor impact on maturity and white mould. White mould is more prevalent in high yield crops regardless of N application, so growers should scout fields and apply control measures when warranted. Applied N increases both bean yield and seed protein with little accumulation in soil N levels. Beans are a shallow rooted crop so N applications should be made in the spring rather than fall to avoid leaching of nitrate-N. Fall soil sampling is essential so that residual nitrate levels are accounted for when fertilizing successive crops.

Recent improvements in inoculant formulations and technology have greatly improved the effectiveness in developing well nodulated soybeans. Inoculation is critical on virgin fields where an inoculated crop has not been grown in the past. Many manufacturers recommend increased rates of inoculant on virgin fields to ensure effective inoculation.

Several conditions may challenge the success of soybean inoculation on virgin fields:

• Initial inoculation remains a challenge on fine textured, wet soils and growers should consider inoculation with a granular inoculant in addition to an on-seed product.
• Excessive levels of residual soil N or applied N as manure or fertilizer may inhibit inoculation. Soybeans will be forced to rely solely on this residual N for growth and yield. When possible growers should avoid such fields for soybeans.
• Some seed treatments may be toxic to seed-applied Rhizobium, so refer to inoculant manufacturer labels to determine compatibility and method of application when using seed treatments (e.g. insecticides and fungicides).

If nodules are not present and soybeans are yellowing at flowering, growers should consider a broadcast application of nitrogen. Apply 50 lb N/ac as broadcast ammonium nitrate or dribble banded UAN solution to minimize leaf burn.

For more information on inoculation, refer to the MAFRI Factsheet “Legume Inoculation” (120-33).

Plant growth promoting rhizobacteria (PGPR) are beneficial bacteria growing within the rhizosphere (i.e. next to the plant roots)⁵¹. PGPRs may promote plant growth through enhanced nutrient availability. Currently the only PGPR registered for commercial use is Bioboost (Registration #2005042A, Fertilizers Act). Bioboost, (Delftia acidovorans) is applied as a seed inoculant for canola.