Soybeans can biologically fix 50 to 60 per cent of their nitrogen, with the rest coming from soil reserves. Bradyrhizobium japonicum is a bacteria specific to soybeans that causes nodule development on plant roots, working symbiotically with the soybean plant to fix nitrogen. Because this rhizobium is not native to Canadian Prairie soils, soybean growers must initially introduce it by using inoculants. After a certain amount of time growing soybeans on the same field, populations will build up in those fields.
Trials led by the Manitoba Pulse and Soybean Growers (MPSG) have confirmed what has been proven in many other long-time soybean growing areas: double inoculation is the best strategy to ensure good nodulation and yields the first time soybeans are grown in a field. After that, MPSG suggests that growers follow its single inoculation checklist to determine if they can drop to a single inoculation. Research is underway to test what would happen if inoculants were not applied during one production year, but recommendations are not finalized.
There are many different inoculant products available in liquid, granular or powder form. The choice of inoculant depends on your situation; your field history, soil type, environmental conditions and application equipment.
Liquid or granular?
Seed-applied liquid inoculants are applied on the seed before seeding. Granular inoculant is generally applied in-furrow and research has shown it to be less affected by excess moisture or dry conditions.
Although the final choice is individual to each farmer, says MPSG production specialist, Cassandra Tkachuk, “In the cases of inoculant failure and nitrogen-deficiency that I have encountered, it appeared to be related more to liquid formulations rather than granular. But we certainly wouldn’t discourage use of liquid inoculants,” she says. “In some cases, a farmer might only be able to put down liquid with the seed, if choosing to single inoculate and not have the capability to put down only granular.”
Granular products allow growers to plant into difficult soil conditions such as low organic matter or dry conditions, because it maintains its moisture content, which helps preserve the rhizobia, says Matt Pfarr, technical manager for Lallemand Plant Care, which develops, manufactures and markets yeast, bacteria and fungi products. That said, granules require more precise equipment calibration than seed-applied liquid inoculants, so both products have a fit for specific situations.
“We’ve definitely talked with some producers that only use granules when they feel they have to,” says Pfarr. “When they have a field that is difficult for them to see nodulation, or they typically have poorer plant performance, they’ll use granular for an inoculant because it’s a tried and true method.”
Liquid is tried and true, and more convenient for some growers, he adds. “Having their local retail apply the liquid to the seed and have seed ready to plant is an advantage,” he says. “But if they’re in the Western Prairies of Canada, they are probably using air seeders, so they have the ability to put down usually two or three forms of dried products One of them is always the seed followed by a dry fertilizer, and then followed by a granular inoculant.”
If you’re using a planter, you have an option to do a liquid in-furrow with a liquid kit. “That can be a way to cost effectively deliver liquid inoculant diluted to about twenty litres per acre in the furrow as well,” says Pfarr. “They can back up an on-seed application of liquid as well, and get them to a double-inoculant application.”
Other things to look at would be co-factors of inoculants, says Pfarr. For example, iron helps plants to get going early. “If growers have had problems with iron-deficiency chlorosis in the past, they could look at a chelated liquid fertilizer in-furrow to make sure that that plant can produce chlorophyll to feed itself and feed the rhizobia carbon to begin the relationship,” says Pfarr. “That would be one example of how a nutritional deficiency, when it’s corrected, can help get the relationship with the rhizobia activated and end up with earlier nitrogen production and more nitrogen at the end of the season.”
A learning curve
While soybeans are becoming established in Western Canada, there’s still a learning curve for new growers.
“I usually share with growers that they definitely want to scout on early years of production,” Pfarr says. “If they’re going to grow peas and soybeans in the same field, that soybean plant isn’t going to benefit from a history of peas or chickpeas on that field because they all have separate species of rhizobia. So, I stress double inoculation for several times they grow soybeans and to make sure that each of those times they have a significant number of nodules per plant and are happy with nodulation.”
Newness of the crop, the length between crop years for the same pulse, fields that have lower organic matter, a pH outside of 6.0 to 8.2, or drought conditions or flooding would be the main markers for a grower that the field will benefit from a couple of placements of fresh rhizobia, says Pfarr.
The MPSG On-Farm network helps growers establish replicated field trials and has been conducting double- versus single-inoculation trials for many years. Two out of 32 site years tested from 2013 to 2018 showed a positive yield response to double inoculation versus single.
“In other words, these trials show a six per cent probability to-date of having positive yield response to double inoculation versus single,” says Tkachuk, adding the trials were done with whatever products each grower was using and was a mix of liquid or granular only versus a double inoculation of liquid and granular together.
That said, single versus double inoculation does require more testing under Western Canadian conditions. “We are not advising farmers to stop inoculating,” says Tkachuk. “We are encouraging them to test this in replicated trials.”
Pfarr advises growers to continue to use at least a single form of inoculant considering the new technologies such as new species and highly efficient strains of rhizobia when it comes to nitrogen production for the crop.
Generally, a rescue application of nitrogen in-season can be used up until the R2 or R3 growth stages, but whether there is any value in applying additional inoculant in-season is research that’s in its infancy. The only available data right now is from some preliminary work done in Brazil, which demonstrated a benefit to applying rhizobia at flowering. “As the plant entered reproductive stage it was asking for another flush of rhizobia because nodules last four to six weeks,” says Pfarr. “That’s not an issue for shorter-season pulses like peas, but soybean can go through two phases of nodulation, after emergence and when they start to flower or pod in July. It may be interesting in the future to look at an aerial or foliar application that would target soil placement of rhizobia at that time to freshen up the amount of rhizobia, but we’re still a few years away from something like that.”