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Why nodulation fails

When nodules don’t form, yield will suffer. 
These recommendations could stop fixation failures

Nodulation failure can be disastrous to crop yields. In some cases, yield may be salvaged. In other cases, it will be too late. Two experts offer recommendations to avoid nitrogen fixation failures.

Pre-farm failure

Nitrogen fixation failures can occur at three points, says Garry Hnatowich, research director at the Irrigation Crop Diversification Corporation at Outlook, Sask. The first is at the point of manufacture, an occurrence that is extremely rare in Canada since inoculants are monitored for quality according to federal regulations.

“Frankly, this is not a major concern,” says Hnatowich. “Reputable inoculant companies all adhere to rigid quality-control procedures and a failure at the point of manufacture is extremely rare, but has occurred.”

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Often, he says, in this situation, product has been damaged in transport from the point of manufacture to the retail distributor.

Failure can also occur at the retail level while in storage, says Hnatowich. “If the product is held in unheated sheds, and night temperatures in early spring drop below freezing then rise above freezing during the day, this freeze-thaw cycle can adversely affect the quality of the inoculant,” he says.

Problems can also occur if the product is stored adjacent to loading bay doors or in any other areas that expose the product to direct sunlight. “Again reputable inoculant companies advise their clients on how best to handle and store the products,” says Hnatowich. “Failure at this point is also rare.”

On-farm failure

The most likely source of problems is on the farm. Diane Knight, a soil science professor at the University of Saskatchewan, says there are a number of reasons why inoculation fails on the farm.

The first problem stems from choosing an inappropriate species or strain of rhizobium for the crop. Improper storage can also lead to failure, she says. Peats and granules should be stored in commercial packaging according to the directions on the packages.

“The rhizobium are live organisms that are susceptible to storage conditions, especially high heat, and sometimes freezing,” says Knight.

Soil conditions are important, too, she says. Seeding inoculated seed or granules into dry soil can lead to failure since moisture is necessary to keep the rhizobia viable. On the other hand, seeding into waterlogged soils is not a good idea either. “Too much water will cut off the oxygen supply to both the nodules and the roots affecting nodulation and proper plant growth,” she says.

Soil that is too acidic can also create problems. Although some rhizobia are adapted to acid conditions, Knight says she’s not sure if they are used in commercial inoculants.

Saline soils can affect both survival of the rhizobia and growth of most legume plants, she continues, as can inadequate soil fertility. “Phosphorus, potassium, iron, molybdenum, manganese and calcium are especially important for either nodule formation or for the proper activity of the nitrogen-fixing enzyme, nitrogenase,” she says.

Finally, soil-borne diseases can affect root growth, especially root hairs. When this happens bacteria enters the root to initiate nodulation through the root hairs.

To avoid incompatible soil conditions, says Knight, careful site selection is key.

“Fertilize with phosphorus and potassium fertilizer as needed according to soil test recommendations,” she says. “If roots are nodulated, but the interior of the nodules are white or cream instead of pink or red it could indicate an iron deficiency.”

If this occurs, test the soil for iron and other micronutrients and amend as recommended. “The red coloration is a compound called leghaemoglobin, which is essential for regulating the oxygen supply in the nodules,” says Knight. “If oxygen is not regulated the nodules will be unable to fix nitrogen.”

Be sure to store inoculants according to the manufacturer’s recommendations, and apply at recommended amounts. “Do not under-apply to save money,” says Knight. “Also, follow instructions carefully for applying them.”

Finally, buy fresh inoculants each growing season. While it may be possible to store some formulations from one season to the next there is no easy way to determine whether or not an inoculant is still viable. “Most inoculants — maybe all — have an expiry date that is usually nine to 12 months after production,” says Knight.

When nodulation fails

If you suspect failure, you should first contact your retail agronomist and the inoculant company’s representative, says Hnatowich.

How do you know if nodulation has failed? Indicators include poor vigour, and yellowing chlorotic plants that develop approximately six weeks after planting.

Carefully dig up some roots, suggests Hnatowich. Then, gently wash them in a bucket of water. Be careful; it doesn’t take much to knock nodules off the roots. If there are numerous nodules of good size, but they are not reddish in colour when cut, then the inoculant is not likely to blame,” he says.

“There could be a contributing plant health problem that’s interfering with effective fixation,” he says. “If few or no nodules are present then you may have an inoculant failure claim.”

The bad news is that by the time visual problems are apparent, yield potential has already been lost. “Just how much yield has been lost depends on the stage stress appears and on the remaining growing season conditions,” says Hnatowich. “There is no effective way to inoculate the established crop.”

The only remaining save is a top-dressed application of nitrogen. “In most situations an application of 50 to 60 pounds of nitrogen per acre is suggested,” says Hnatowich. “Higher rates may only extend the maturity by keeping the crop in a vegetative state longer than wanted and don’t guarantee higher yields. This is usually a ‘salvage’ operation, an attempt to salvage enough yield to break even or better.”

“However it’s a bad situation, and a catch up and pray situation that’s unlikely to burst any bins at the end of harvest,” concludes Hnatowich.

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