Soybean inoculation should double on virgin ground

Doubling inoculation where the crop hasn’t been grown before adds cost, but it works; added nitrogen doesn’t

When it comes to growing soybeans, doing a little bit more with commercial inoculants can pay huge dividends at harvest time, especially on ground that’s never grown soybeans before.

Garry Hnatowich, a research agronomist at the Irrigation Crop Diversification Centre near Outlook, Sask., said proper soybean inoculation is critical to getting the crop off to a good start.

“Bradyrhizobium japonicum (the inoculum species used on soybeans) is not native to Canada or to North America, so we have to introduce it,” Hnatowich told an audience at CropSphere in Saskatoon.

“The only way to introduce it is through commercial inoculum.”

Label recommendations on commercial inoculants normally target a minimum of 100,000 bacteria per seed at the time of seeding.

However, recommended inoculation rates should be doubled on first-time soybean ground to ensure adequate nodulation.

“That sounds like a big number,” Hnatowich said.

“But I guarantee you that if you were depending on 100,000 bacteria on each and every seed, you would have an inoculum failure. It’s simply not enough.”

Label rates are typically based on an assumption that bacterial populations will multiply five to eight times within the first few weeks of seeding.

However, under less than ideal conditions, multiplication might not occur as quickly as expected.

“You need millions of bacteria present to get effective nitrogen fixation,” Hnatowich said.

“Those are the numbers we want.”

“What we’re looking at is following the Manitoba recommendations of double inoculation on virgin grounds,” he said.

“Use a good quality liquid in conjunction with a 1x rate of granular inoculum, if you can.”

Once it’s established in the soil, populations of bradyrhizobium japonicum will overwinter in prairie soils, Hnatowich added.

Therefore, after two years of good fixation and double inoculation, it is reasonable to assume that single rate inoculation should be adequate to achieve optimal yields.

The relationship between inoculation and the benefits of supplemental nitrogen applications has been the subject of numerous research projects.

In Saskatchewan, Hnatowich was a co-investigator in a three-year, multiple-site soybean fertility study conducted at Indian Head, Outlook and Melfort.

The objective of the study was to investigate soybean yield response to dual inoculation, nitrogen fertilization and the potential interactions between the two factors.

Seed used in the trials was commercially treated using a well-known liquid inoculant as well as a granular inoculant product.

Plots at each location received various treatments.

The least intensive treatment was a liquid-only inoculation, applied at a 1x rate, with no granular inoculant and no supplemental nitrogen.

The more intensive treatments included liquid inoculant at a 1x rate, granular inoculants applied at 1x, 2x or 4x rates and 50 pounds of supplemental nitrogen either seed placed, side-banded or dribble-banded.

Preliminary results from Indian Head and Outlook point to a significant yield boost by applying double inoculants (granular and liquid).

Conversely, when seeded crop has been double inoculated, there is a relatively minor yield benefit to be gained by applying supplemental nitrogen.

“The bottom line? Where we’ve got effective nitrogen fixation occurring, there is no positive effect from nitrogen additions,” Hnatowich said.

“(In other words), there’s no strong evidence that we require additional nitrogen over what’s already being provided through fixation.

“We do see (a boost) in early season vegetative growth, but rarely, rarely are we finding that it’s translating into (higher) seed yields.”

Hnatowich noted that results from the study are preliminary and data from some test sites has yet be fully analyzed.

Furthermore, in the event of an inoculation failure, the addition of supplemental nitrogen may have a positive impact on yield.

Hnatowich also spoke about the relationship between phosphorus and seed yield in soybeans.

Citing soybean fertility studies conducted in the United States and Canada, Hnatowich said the economic benefits of applying P205 to soybean crops during the production season appear to be minimal.

“Generally, if you look at the traditional growing area of the U.S. corn belt, phosphorus fertilization in the year of soybean production is a non-issue,” he said.

“It’s not done. In fact, they don’t put any fertilizer down with their soybeans whatsoever. What they’re doing instead is putting high amounts down on the previous corn crop and then using soybeans the following year to pick up any fertilizer that the corn did not utilize.

“That’s been a very effective strategy for them … because soybeans are known (for their ability) to scavenge residual soil nutrients, in particular phosphorus.”

Although soybeans have the potential to use a large amount of phosphorus — up to 0.8 lb. per bushel of seed produced — actual phosphorus utilization can vary significantly depending on soil types and textures.

In some cases, phosphorus applications can have a negative yield response, depending on soil characteristics, application rates and fertilizer placement.

One study conducted in Saskatchewan in 2015 examined the yield benefits of supplemental phosphorus, either seed placed, side-banded or surface broadcasted, applied at zero lb. per acre, 20 lb., 40 lb. and 80 lb.

Notably, seed-placed phosphorus applied at the 40 or 80 lb. rates had a negative impact on yield, supporting the notion that soybeans have a sensitivity to seed-placed fresh commercial fertilizers.

“In fact, they don’t really like fresh fertilizer additions,” Hnatowich said.

“Soybeans like to have phosphorus around, but they don’t like fresh commercial fertilizers. They’d rather have a mellowed out fertilizer.”

Saskatchewan farmland is generally low in phosphorus, suggesting that repeated soybean production will require supplemental phosphorus, presumably applied over the long term.

Iron chlorosis can be a problem in some soybean producing regions of Western Canada, particularly in the Red River Valley and especially on high calcium, high pH soils that are prone to poor drainage or flooding.

“Soybeans are great for having wet feet. They’ll stand under flooded conditions for three days and they will survive,” he said.

“They’ll turn yellow but they will survive, and iron chlorosis will often show up.”

Soybeans suffering from iron chlorosis will typically have strong green veins within a plant that is predominantly yellow or chlorotic.

Producers who grow soybeans on slightly saline, high pH soils should pay close attention to soybean genetics, opting for varieties that are tolerant to iron deficiency chlorosis.

Where necessary, foliar application of iron may be used as a rescue treatment.

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