Description
Well-nodulated soybean plants are essential for producing the nitrogen needed to support high yields, but the only way to know if the crop is on track is to get out of the cab and dig some plants.
In this episode of Soybean School, RealAgriculture's Bernard Tobin talks with Lallemand Plant Care field solutions specialist Nikki Burnett about when growers should be evaluating nodulation, what they should be looking for below ground, and how good nodulation can pay dividends later in the season.
Burnett encourages growers to begin checking roots early, around the V3 to V4 growth stages, when nodules should already be forming. A second assessment at the R2 to R3 reproductive stages is equally important because that's when nitrogen demand and nitrogen fixation are at their peak.
When assessing plants, Burnett recommends looking for 10 to 20 healthy root nodules, although more is even better. Larger nodules clustered around the crown are a good indication that a seed-applied inoculant has been effective, while Manitoba growers using in-furrow granular inoculants should expect to see more nodules on the lateral roots because of the different application method.
Growers should also split open a few nodules to confirm they're actively fixing nitrogen. "They should have a bright pink centre once you crack them open," says Burnett. "That's the key for making sure that these are looking good."
Good nodulation has a direct impact on yield potential. Burnett notes that research suggests plants with 10 to 20 healthy nodules can achieve at least 90 per cent of their yield potential. She also recommends pairing below-ground observations with what the crop is telling growers above ground. Yellowing, particularly on older leaves, can signal nitrogen deficiency and poor nodulation, while healthy green plants with few nodules may simply be accessing residual nitrogen already present in the soil.
To encourage strong nodulation, Burnett says growers should pay close attention to higher-risk fields, including those that have experienced significant drought or flooding, where native Bradyrhizobium populations may have been reduced. She also stresses that inoculant quality matters. "Not all Bradyrhizobium are created equal," she says, noting that researchers use specialized tests to identify bacterial strains that are most effective at fixing nitrogen and supplying it to the soybean crop.
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