Adequate root nodulation is critical for filling soybean pods with seed, as there’s a strong correlation between the number of nodules on soybean plants’ roots and yield.

But in a dry year, the number of nodules might not matter quite as much, as soybeans can compensate for reduced nodulation by producing larger nodules.

As part of this Soybean School episode, we grab a shovel and head to the field once again with Cassandra Tkachuk, production specialist with Manitoba Pulse and Soybean Growers, to take a look at what’s happening below the soil surface on the roots of drought-stressed plants.

“Our research has shown you want at least 10 nodules per root system for maximum yield potential,” she explains. “That particular study didn’t look at nodule size, but under drought conditions, nodule size can actually play a role, where the larger the nodule, the less susceptible it is to drought.”

“We’re seeing a lot of that this summer, which is a good sign,” continues Tkachuk, in the video below.

She emphasizes the importance of assessing root nodules each year, regardless of growing conditions, past soybean performance, or inoculant practices. The ideal time to assess nodulation is at the R1 (beginning bloom) stage, as there’s still an opportunity for a rescue nitrogen treatment.

In addition to a shovel to gently dig out each plant, a pail of water may be needed to soak the roots to remove the attached soil, especially in dry clay soils. Tkachuk demonstrates how to cut each nodule open to assess the colour inside. If nodules are a pinkish-red inside, the Bradyrhizobium bacteria in the nodules are actively fixing N.

MPSG’s Cassandra Tkachuk discusses nodulation, inoculants, and the impact on drought on nitrogen fixation as part of this Soybean School video:

Related Soybean School episode: Rescuing soybeans suffering from poor nodulation

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