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Inoculant: Finding the right formulation

Liquid, peat or granular? Choose the best one for your fields

Inoculants can offer tremendous value to growers at a reasonable price. Under ideal conditions, all three formulations — liquid, peat or granular — can be equally effective. As conditions deteriorate, though, differences can occur.

The different types of inoculants refer to the substrate that the rhizobium bacteria are applied to, says Dr. Diane Knight, soil science professor at the University of Saskatchewan.

Liquid inoculants are broth cultures that are sold either refrigerated or as frozen concentrates. Usually, they are mixed with water and sprayed into the seed furrow at planting. “Liquid inoculants are probably the least popular because they need to be refrigerated during shipping and storage,” says Knight. “In Western Canada, liquids are not as common as peat and granular inoculants.”

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With peat inoculants the bacteria adhere to a commercial grade, finely ground peat moss. “The peat provides a relatively inert substrate that the bacteria can survive on in high numbers,” says Knight. “Peat inoculants sometimes are manufactured with a ‘sticker’ substance added to adhere the peat inoculant to the seed. With non-sticking formulations the farmer can add a sticker separately. The powdered peat is applied directly to the seed at seeding so that the bacteria are in direct contact with the seed.”

Granular inoculants are small pellets, usually made of an inert clay — although other substances have been used — that the bacteria are applied to. Similar to peat, granular inoculants provide an inert stable substrate that supports the bacteria in high numbers. Granular inoculants are applied to the soil rather than to the seed.

Choosing the right rhizobium

Crops are very selective about the type of bacteria they use to form nodules that result in the desired nitrogen fixation. Field peas, lentils and faba beans, for instance, require Rhizobium leguminosarum species. Chickpeas require Mesorhizobium cicero, dry beans require Rhizobium phaseeoli and soybeans require Bradyrhizobium japonicum.

“Apply a pea inoculant to soybean and no infection of bacteria will occur,” says Garry Hnatowich, research director at the Irrigation Crop Diversification Corporation at Outlook, Sask.

Knight agrees. “Purchasing the correct inoculant for a particular pulse crop is the most important decision — makes far more difference than whether it is a peat/granular or liquid inoculant,” she says. “Applying an inoculant for soybean onto pea will have no benefit. It is the same as applying nothing, just more expensive.”

Which inoculant works best?

Powdered peat-based inoculants were first introduced for sale in North America in 1897, says Hnatowich. “Up until about 25 years ago, they remained the only commercial formulation available to growers,” he says.

“This formulation was awkward to apply with accuracy, dusty and had poor adhesion with the seed they were applied to,” says Hnatowich. “Many farmers resorted to mixing in ‘glues’ to help the inoculant stay on the seed, such as powdered milk, honey and even cola.”

“Liquid inoculants alleviated much of the application difficulties,” he says. “They could be conveniently metered and applied as seed was augered into the truck or into the seeder.”

As the number of seed/input tanks on seeders increased, more granular inoculants were developed. Now application rates can be easily calibrated, and they offer a positional placement within the seed-row that can be beneficial, says Hnatowich.

“The three types differ in the way the bacteria are delivered to the system,” says Knight. “Because they are delivered differently, the equipment needed is different. The three formulations give the farmer a choice depending on the equipment he/she has available.”

Liquid vs. granular

With the improvements to formulations, Hnatowich says that all three types of inoculants can be equally effective. “Each can still have drawbacks, however,” he says.

Liquids were once notorious for less than desirable performance for two main reasons. “For one, the bacteria can be killed by desiccation and these formulations were designed to dry quickly once applied to seed, so bridging of seed at planting did not occur,” says Hnatowich. “This rapid drying resulted in reduced bacteria load on each seed and this was further exacerbated if seeding delay occurred.”

On top of that, liquids were often applied to seed that had been previously treated with a seed treatment. While seed treatments are designed to have an impact on microorganisms, like fungi, they can also decrease the bacteria load.

“The development of enhancers additives has assisted with the desiccation concerns and newer seed treatments entering the market are, generally, more inoculant friendly,” says Hnatowich.

Liquid formulations can still be problematic if the seed is planted into less than desirable soil moisture conditions. Peat-based inoculants are still messy, which makes them difficult to apply, although desiccation is less of a problem since peat has inherent moisture content that helps keep the bacteria from drying. The peat also acts as a physical barrier between the seed and the seed treatment.

“Granular inoculants have been considered the ‘Cadillac’ of formulations,” says Hnatowich. “Applied in kilograms rather than grams or millilitres, the amount of bacteria that can be applied per unit area is higher. Seed treatment compatibility is minimized because most inoculant prills are not in direct contact with seed treatments. And moisture content of the prills is also such that desiccation is minimal. Bridging of the inoculant can occur, so a limited number of bags should be used in the tank, which means frequent stops during seeding are required to replenish the inoculant.”

In general, though, granular formulations are said to be the best choice. In fact, numerous studies have shown them to be superior to the other formulations. “Unfortunately, there is a price to be paid, as granular inoculants are the most expensive inoculant formulation,” says Hnatowich.

“With effective nitrogen fixation occurring in annual legume grain crops the price of inoculants is a fraction of the cost that commercial fertilizer would be in order to achieve the same yields,” he advises. “Therefore, treat them for what they’re worth, not what they cost.”

Melanie Epp is a freelance farm writer based in Guelph, Ont.

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