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Using rhizobium inoculants

High yields for pulse crops depend on using the right type of inoculant at just the right time, to get as much bacteria into the soil as possible

Rhizobium inoculants help peas and lentils fix nitrogen, reducing or eliminating the need for applied nitrogen. Success hinges on getting as many live rhizobium bacteria into the soil as possible.

“You want to be able to maintain them as viable bacterial cells,” says Dr. Fran Walley. Walley is a soil science professor at the University of Saskatchewan.

Proper storage and handling keep rhizobia alive and thriving. Rhizobia are vulnerable to ultra violet rays, so inoculant should be stored away from sunlight. Heat and wind will also dry out and kill rhizobia.

Soil tests can indicate potential problems.

“Inorganic nitrogen — nitrate, for instance — can inhibit the nodulation, nitrogen fixation process. So you want to have a pretty good notion of what your soil test nitrogen levels are,” says Walley. If the roots don’t develop any nodules, there may be too much nitrate in the soil.

Choosing the right product

Farmers should consider convenience, manpower, formulation compatibility and soil conditions when choosing a formulation.

All three formulations can work fine for peas and lentils, says Danielle Fletcher, field agronomist with Becker Underwood.

“They’re all medalists at the Olympics, but basically your granular is gold medalist because he’s altitude trained and can withstand a bunch of different conditions,” says Fletcher.

Fletcher says peat or granular inoculants are better for moderately acidic soils, hot and dry conditions, loosely rotated ground, and other adverse conditions because they’re more stable than liquid formulas. Liquid inoculants aren’t recommended for soils with low indigenous rhizobium populations, either — for example, previously flooded land or land that has never been inoculated.

Granular inoculants can be particularly effective in soils with low rhizobium numbers, Walley says.

“The advantage of the granular product is that it tends to result in nodules forming on more of the lateral roots. Whereas a peat-based powder — a seed-applied inoculant — generally forms nodules right around seed placement. And it seems that if the levels of rhizobium in the soil are low… there is a real advantage to getting some nodules on those lateral roots. And that’s where the (granular) products look really, really good,” says Walley.

Walley says farmers who have used inoculants for several years have resident rhizobia that will form nodules on lateral roots. In these soils, farmers may see the same benefits from using less expensive seed-placed peat or liquid inoculants, which form nodules around the root crown.

Fletcher says application convenience is driving the shift to granular inoculants. Farmers simply load the granular into the air seeder rather than applying it to seed. Fletcher says the inoculant does need its own tank, as mixing it with fertilizer harms the bacteria.

Farmers using a peat inoculant can cut the dust by dampening the peat with water while applying it to the seed. Fletcher says some peat inoculants have a built-in adhesive that is enhanced by water.

Farmers using a granular inoculant don’t need to check whether seed treatments are compatible with the inoculants.

“But when you’re dealing with seed applied (inoculants) — so either the peat or the liquid — you have to watch because there are getting to be more things put on seed. And you need to understand the implications of those interactions and how they’re going to affect the bacteria,” says Fletcher.

Fletcher says Becker Underwood staff test for compatibility with many registered seed treatments, including some registered fertility seed treatments. But the company doesn’t test every combination of products that can be put on the seed as there are too many possible combinations.

Application timing

Farmers using pea and lentil liquid inoculants only have two or three hours to get the seed into the ground. If farmers leave liquid-treated seed sitting too long, they’ll need to re-inoculate it. Fletcher says planting inoculated seed that has sat too long can hurt crop performance.

“I’ve seen that with liquid in some situations, where you start out in the beginning of the field, and it’s fine, and you can literally see almost where they’re filling up with seed and doing the new inoculation again.”

Once farmers have inoculated seed, the best thing they can do is get the seed into the ground as soon as possible, says Fletcher.

Pea and lentil liquid inoculants have the shortest safe planting interval, and peats are also time sensitive. Granular inoculants are not time-sensitive as the physical separation of the inoculant and the seed provides an adequate buffer. Soybean inoculants generally have a longer safe planting interval as the rhizobium species that inoculates soybeans is hardier.

Farmers may try to manage risks by, for example, using two different peat inoculants, applied to the seed at a 60 per cent rate. Fletcher says that idea has merit, but a better method would be to combine a granular inoculant with a seed-applied inoculant.

Inoculate when resident rhizobium plentiful

Farmers who have been inoculating the soil for years may be tempted to skip the inoculant with their next pulse crop. But native rhizobia can be inefficient nitrogen fixers, so farmers need to saturate the soil with fresh bacteria with each pulse crop, says Fletcher.

Research shows resident rhizobia’s abilities to survive and fix nitrogen vary tremendously.

“The genetic material that’s coding for nitrogen fixation can actually be lost from rhizobium. And so you want to continue to use a fresh, good inoculant every time you’re growing a pulse crop or a legume,” says Walley.

Companies are always hunting for better rhizobium strains with desirable characteristics, such as performance in cool soils or acidic soils, Walley says.

Fletcher compares inoculants to insurance.

“If you don’t have good nodulation, then you’re looking at applying a bunch of nitrogen fertilizer, which is a whole lot more costly than your cheap insurance for inoculants that are in the single digit dollars per acre.” †

About the author

Field Editor

Lisa Guenther is field editor for Grainews based at Livelong, Sask. You can follow her on Twitter @LtoG.

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