Many options when it comes to inoculants for pulse crops DEC 7 2020 | Producers | Agronomy and Blog Post
By Mark Olson
Pulse crops as legumes are a valuable component in diversifying Western Canadian cropping systems. Their ability to fix nitrogen from the air reduces the need for nitrogen fertilizer, lowering the carbon footprint as well as making a more sustainable cropping system overall.
When pulse crops are inoculated with a species-specific bacteria (rhizobia) nodules are formed around the root hairs of the host plant. This is a mutually beneficial relationship where the plant provides the rhizobium with sugars and mineral nutrients, and in return the rhizobium provides nitrogen to the plant.
Nitrogen is not only the nutrient required in the largest quantity by all crops but forms the basis for their total seed protein content percentage. Inadequate amounts of nitrogen will not only limit plant growth and yield but will also result in lower than average seed protein. For these reasons, proper inoculation is mission critical for Alberta growers.
When successfully inoculated (dependent upon pulse species), the crop can fix anywhere from 40 to 90% of its own nitrogen requirements. Faba bean has the highest nitrogen fixation percentage (90%) of all annually grown legume crops. In contrast, dry bean would have one of the lowest percentages (40%) with all other species falling somewhere in between.
Inoculants are the product carriers for the rhizobia and come in three basic formulations: peat, liquid and granular. The peat-based formulation is a mixture of a finely ground peat combined with the rhizobia and applied directly to the seed with a sticking agent. Liquid inoculants are typically packaged in a plastic bladder with the rhizobia contained within and applied directly to seed either shortly before seeding, at the time of seeding, or in the seed row with an on-the-go liquid applicator. Granular inoculant can be either peat or clay-based granules applied in the seed row or in some instances, side-banded within a separate granular inoculant tank.
Inoculant types are strongly affected by their soil environment, how they are stored, and how they are applied. No matter which type of inoculant formulation is used, careful attention must be paid to the rhizobia. As rhizobia are a living organism, they will dry out when stored at too high of a room temperature or become completely ineffectual when exposed to direct sunlight. As well, the number of live bacteria will naturally decline over time. Growers are encouraged to check the product labels for the expiry date of their inoculant.
Once applied to the seed, rhizobia populations in the peat and liquid formulations decline more rapidly than in the granular particle. For this reason, they need to be seeded as quickly as possible for optimal nitrogen fixation. As well, growers should make sure when applying a seed treatment to check the product label for compatibility with multiple inoculants. Some products may have to be applied sequentially (i.e. seed treatment applied first and allowed to dry before applying the inoculant). Do not apply inoculant in combination with a fertilizer product as this is high risk. The salts contained in the fertilizer will cause a decline in rhizobia numbers. Under dry, hot and low pH soils (harsh environmental conditions), research has shown that granular inoculants provide more uniform and consistent rhizobia populations than with peat and liquid formulations. When environmental conditions are optimal, no differences between formulations have been observed in research trials. The formulation a grower uses is therefore a matter of personal choice and economics.
Growers can check for nodulation of their pulse crop by carefully digging up their plants (not pulling the plants up like a carrot) 4-5 weeks after seeding. If a peat or liquid formulation was applied directly to the seed, the nodules will form close to the crown and main tap root. A granular inoculant pattern has some nodules on the main tap root as well as scattered throughout the lateral roots.
Growers or agronomists wanting to do more rigorous nodule assessment can find a guide at https://www.saskatchewan.ca/business/agriculture-natural-resources-and-industry/agribusiness-farmers-and-ranchers/crops-and-irrigation/soils-fertility-and-nutrients/nodulation-and-nitrogen-fixation-field-assessment-guide .