Nodulation No Time to Nod Off for Producers

What growers should be looking for to help their pulse crops

by Geoff Geddes

Oxygen may be free for humans – but nitrogen for pulse crops comes at a price. That is why nitrogen fixation is so important in reducing fertilizer costs, and providing other benefits in the process. But those crops need help before fixation can take place, and for that, the nod goes to nodulation.

“Nodules form on the roots of legume crops when an appropriate species of bacteria invades the root through the root hairs of the plant,” said Dr. J. Diane Knight, Saskatchewan Ministry of Agriculture Chair, and Professor with the Department of Soil Science at the University of Saskatchewan.

The Fix Is In

The bacteria multiply in the root tissue and form a structure, a nodule, where symbiotic nitrogen fixation occurs. The nitrogen fixation process takes nitrogen from the soil atmosphere and fixes it into a form of ammonium that the plant can use to make proteins and other compounds containing nitrogen.

As with any healthy relationship, the plant and the bacteria both benefit from this process. While the plant gets nitrogen in a form that it needs, the bacteria derives energy from the plant to maintain the nodule and drive nitrogen fixation. Thus Knight emphasizes that “in order for pulse crops to fix nitrogen from the air, they need to be properly nodulated.” When that occurs, the benefits to producers are numerous.

Lending a Hand to Lower the Footprint

“By far the most important nutrient in plant production is nitrogen,” says Dale Risula, Provincial Specialist, Special Crops with the Saskatchewan Ministry of Agriculture. “Nitrogen is added to most crops artificially through fertilizers,” said Risula. “Unfortunately it takes a lot of energy to produce the nitrogen required in crop production. That just adds to the carbon footprint that is a major environmental issue these days.”

By reducing the fertilizer required, nitrogen fixing through nodulation shrinks that footprint while expanding producer wallets, as the price for nitrogen produced this way is hard to beat: free.

As a bonus, the secondary effect of fixation is the nitrogen residue often left in the soil which can give next year’s crop a head start. So if nodulation is the goal, how can producers offer it a head start?

Get Wet

Like pulses themselves, nitrogen fixation needs oxygen and water to succeed. And though it is hard to overdose on the former, the latter can be tricky. “Overly wet or dry conditions can both be a problem. Proper fixation and survival depends on the right balance of oxygen and moisture,” says Risula.

Perhaps the biggest factor is the presence of the proper species of Rhizobium in the root zone of the appropriate species of pulse crop. “The symbiotic relationship between plant and bacteria is very specific, so the correct species of both the legume and the Rhizobium must be present,” says Knight.

For example, Knight explains, “The Rhizobium for peas is a different species than the one for chickpeas. You must ensure that the proper species of Rhizobium is there by applying the correct Rhizobium inoculant.” 

A Step In the Right Direction 

And as with most things in life, if all else fails, read the directions. “There are peat, liquid, and granular types of bacteria, each with its own method of application,” Risula says. “Also, not all fungicides and insecticides used for seed treatment are compatible with all bacteria, so they should not be used together in some cases.”

Fortunately, all of these instructions are on the label for each product. It is just a matter of spending a few extra minutes reviewing them, time that could prove priceless in the long run. Though following the directions will improve your prospects, there are no guarantees.

It Is What Is Inside That Counts

“If your plants look healthy and vigorous, chances are the nodulation was successful,” says Risula. “But if they are yellow or weak, you need to dig up the plant carefully to avoid shearing off the nodules, wash the dirt from the roots and take a closer look.” 

According to Knight, the nodules should have a creamy whitish-pink appearance and be visibly pink or red inside.“The red coloration indicates that the nodules are active,” explains Knight. “If young plants are yellow and nodules cannot be found, it is probably best to add some nitrogen fertilizer so that at least a crop can be harvested, although the plants will not fix nitrogen. For annual pulse crops they cannot be inoculated after the crop has emerged.” 

Knight says different crops will have different numbers, sizes, and shapes of nodules. Lentils have two lobed nodules whereas chickpeas tend to be single lobed and rounder. “The size and shape do not matter much as long as some active nodules are present.”

What really matters to producers is optimizing nodulation and keeping fertilizer investment to a minimum.