Inoculants for Optimal Nodulation

November 15, 2018

Whether you’re a veteran soybean grower or relatively new to the crop, an inoculant is one of the key ingredients to your success. It provides beneficial soil bacteria (rhizobia) that encourages the development of nitrogen-fixing nodules on soybean roots, which leads to healthier plants and higher yields.

All inoculants are living organisms and available in liquid, peat or granular formats. This provides growers with choice and flexibility, but it can also lead to confusion.

Syngenta agronomist Doug Fotheringham fields a lot of questions from growers asking which is the best formulation to use. His answer? “Really nothing is better than the other. They all work, and they all work very well,” he says, adding that growers shouldn’t get caught up in figuring out which one is best. A better approach is to make your choice based on the equipment you have on the farm to deliver the inoculant.

Liquid vs. peat vs. granular

Fotheringham notes there are only slight differences between the three formulations. When liquid is seed-applied, it is available right at the crown and recognized quickly by the plant. This allows nodulation to start a little earlier. One downfall is that liquid isn’t quite as robust as the other formulations in harsher conditions.

Peat is also seed-applied, but its properties make it a little more robust than liquid. Ease of application is one drawback – it can be messier and a little more cumbersome to handle. In addition, if rates aren’t quite right, clumping and bridging can result.

Granular is considered the ‘Cadillac’ formulation due to its robustness. Each little prill is a clay-based product with the rhizobia built-in, providing its own feeding source. “That's a benefit because when we're planting into adverse conditions, that prill can feed off of itself for a little bit,” explains Fotheringham.

The downside of granular when comparing to liquid or peat is related to handling – its large bags simply make it bulkier to deal with.

The bottom line is that it doesn’t matter what you use, just make sure to support your soybeans with some form of inoculant.

Two sources cover the bases

One thing Fotheringham does advise is using two sources of inoculant – especially on virgin or newer soybean ground.

“The best-case scenario is to put a 1x rate right on the seed, and then also put a 1x rate of granular inoculant in furrow. It covers all of your bases,” he says. “You've got something right on the seed that's available for that plant to sense and take up. You've also got the granular inoculant in furrow, so that if conditions get challenging, you've still got that granular product there that's going to be able to fight through.”

Fotheringham points out that even if you end up with perfect environmental conditions, you'll have a nice area of nodulation right at that crown where the plant starts its growth. Then when those lateral roots move out, they'll find and pick up that granular inoculant.

“It's like a secondary source, so you'll have nice, even nodulation distribution throughout your roots,” he says. “If you put a 2x rate down and it maybe didn't quite need that, it's still a very minor expense in the grand scheme of things. To me, it's a no brainer because there's no going back once you realize you’ve got nodulation issues.”

There are plenty of other application options that can achieve the same positive results. For example, for growers who have a liquid kit, either on their planter or air seeder, they can easily deliver a 1.5x or 2x rate of liquid inoculant in furrow. Or, they can have 1x rate on the seed, with a 1x rate dribbled in the furrow.

Don’t cut corners

If you’ve been growing soybeans for a number of years, you may be tempted to cut corners and skip inoculant application, but Fotheringham warns against it.

“It's true, the more times that you have a soybean planted on a particular field, the more rhizobia you can build up over time,” he says. “It's important for growers to be thinking about growing conditions. If the crop has gone through a stressful time – too dry, too wet, etcetera – rhizobia numbers may be reduced. That's why you still need to include an inoculant.”

Fotheringham explains that in addition to environmental conditions, field variability can also affect rhizobia survivability.

“For fields that are really variable, from heavy clay to sand, you may want to consider more than just a 1x rate even if you’ve grown beans on that particular field a number of times. In those extreme areas of the field there's not much for rhizobia to survive on.”

Inoculants as insurance

For anyone not convinced of the value of inoculants, Fotheringham suggests an easy experiment.

“Shut off your granular tank for 50 feet at seeding and watch that area in season. You will see a dramatic difference in growth between the plants that have it and the plants that don't. Even on the field that has a history of soybeans, there will be a difference in heights. There will be difference in colour and that, in turn, will result in a difference in yield for sure,” he says.

Fotheringham encourages growers to talk to their local agronomist or rep to figure out how much inoculant you actually need to apply. Historically, the general rule of thumb was eight to 10 pounds, but application rates are really dependent on row spacing.

He sums it up like this: “when you look at the investment that you put into a soybean acre, your costs are significant. In my opinion, the cheapest insurance that you will put down on that acre is inoculant, period.”

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