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Check soybean nodules to track plant health

Soybean Watch: It’s time for nodules to kick in and fix bacteria for the crop.

Tom J. Bechman, Midwest Crops Editor

June 21, 2024

4 Min Read
Nodules on soybean roots
CHECKING ROOTS: Plenty of nodules should be apparent on roots of soybean plants this size, agronomist Steve Gauck says. The nodules host bacteria, which fix nitrogen from the air. Tom J. Bechman

Steve Gauck arrives at the Soybean Watch ’24 field for an early- to midseason checkup with several tools. He makes sure he has a trowel for removing plants, a shovel in the pickup, a copy of the Purdue Corn and Soybean Field Guide in his back pocket for identification, and a drone just in case something is so unusual he wants to scout possible patterns from the air.

Today, the trowel and Purdue guide will suffice. “The first thing I do is stage plant growth,” says Gauck, a regional agronomy manager for Beck’s, sponsor of Soybean Watch ’24. He is based near Greensburg, Ind.

“Once we know growth stage, we know what to expect in terms of development,” he says. “For example, if we find V4 and V5 soybeans and dig them up, we ought to find nodules on the roots. It is the stage where nodules should kick in and supply nitrogen for plants.”

To make sure, Gauck digs several plants and examines roots. Each nodule contains Rhizobium bacteria. If it’s not too wet and bacteria are working properly, nodules should be pink inside.

“That signifies that bacteria are pulling nitrogen out of the air and turning it into a usable form of nitrogen for plants,” he explains.

A close-up of nodules on a soybean plant, with one cut open

Nodules and soybeans

One reason Gauck leans toward a somewhat deeper planting depth is because he believes it gives nodulation an edge, particularly in drier years. “If nodules develop a bit deeper, there are better odds of sufficient moisture for them,” he says.

Related:How to determine early growth stages in soybeans

People sometimes ask if soybeans must have nodules to get nitrogen. “Soybean Growth and Development,” Iowa State University Publication PM1945, recently updated by Mark Licht and other agronomists, notes that soybeans use both fixed and mineralized nitrogen from bacteria, and nitrogen in the soil from fertilizer. The soil N is used by young plants before nodulation begins.

“From this stage on, plants rely most on fixed nitrogen from nodules,” Gauck says. “That’s why we believe starter fertilizer with nitrogen doesn’t always pay for soybeans.”

In fact, the ISU publication notes that the total number of root nodules decreases if fertilizer rate increases. And if fertilizer is added for plants with active nodules, those nodules may become somewhat inactive or inefficient, depending on amount of N fertilizer applied.

“When environmental conditions are right and soybeans are healthy, nodules will do the job and supply what plants need for good yields,” Gauck concludes.

From the field:
Soybeans settle into growing season

A fitful start with late May rains and flooding in some areas will hopefully give way to a more normal growing season. Here is an update from Beck’s agronomists across the Midwest:

In Iowa. “In southeast Iowa, April-planted beans are getting good growth and color. They are at the V4 growth stage. Some post herbicide applications are underway. Beck’s Practical Farm Research data indicates V4 is a good time to apply foliar nutrition. May-planted beans are up and rolling with the punches. They endured some wet weather. Now, we’re getting hot and dry. All crops are looking better every day.” — Greg Shepherd

In South Dakota. “It has been a challenging and long planting season across South Dakota. Numerous rain events compared to last year and cooler weather slowed early planting. Growth stages are from just planted to V3 or V4. Some fields show yellowing from excess water, and I am seeing some iron disease chlorosis, or IDC.

“I am monitoring a field, and it is in the V2 stage. Plants look healthy, with no activity from bean leaf beetles yet. It is currently clean for weeds. We are getting some heat now, and these soybeans should really start to grow quickly. With luck we will get a little rain — hoping the hail stays away.” — Jerry Mathis

In Wisconsin. “We’ve had a slow week for crop development. Many fields seemed to go ‘backward’ with excessive rain and cool, cloudy conditions. This upcoming week is supposed to be much sunnier and warmer. Post herbicide applications are getting going as soils dry out. Root and stem diseases will likely pop up in spots, but at least it will now be in warm and wet soil.” — Joey Heneghan 

About the Author(s)

Tom J. Bechman

Midwest Crops Editor, Farm Progress

Tom J. Bechman became the Midwest Crops editor at Farm Progress in 2024 after serving as editor of Indiana Prairie Farmer for 23 years. He joined Farm Progress in 1981 as a field editor, first writing stories to help farmers adjust to a difficult harvest after a tough weather year. His goal today is the same — writing stories that help farmers adjust to a changing environment in a profitable manner.

Bechman knows about Indiana agriculture because he grew up on a small dairy farm and worked with young farmers as a vocational agriculture teacher and FFA advisor before joining Farm Progress. He works closely with Purdue University specialists, Indiana Farm Bureau and commodity groups to cover cutting-edge issues affecting farmers. He specializes in writing crop stories with a focus on obtaining the highest and most economical yields possible.

Tom and his wife, Carla, have four children: Allison, Ashley, Daniel and Kayla, plus eight grandchildren. They raise produce for the food pantry and house 4-H animals for the grandkids on their small acreage near Franklin, Ind.

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