Wallaces Farmer

Soybean management: Make better decisions by understanding nutrition

Knowing these simple facts about soybeans and nutrients can improve your management — and your yields.

Tom J. Bechman, Midwest Crops Editor

July 25, 2024

5 Min Read
A close up of soybean roots and nodulation
ENCOURAGING NODULATION: Some agronomists believe planting soybeans deeper encourages more nodulation because soil is cooler. Planting depth increased from left to right, and early nodulation also is more prominent on the right. Tom J. Bechman

Do you know why soybeans don’t respond to nitrogen fertilizer in the spring? Do you know which nutrient is more soluble — nitrogen or potassium? Do you realize that if you understood more fully how and when soybeans use nutrients, you could fine-tune your management practices?

Here’s a primer on soybean nutrition, based on Soybean Growth and Development, PM 1945, a publication by Iowa State University. Mark Licht, Extension cropping specialist for corn and soybeans at ISU, helps walk you through this refresher. These explanations could help you fine-tune your soybean management plans:

Where do plants get nitrogen? A large part comes from bacterial nitrogen fixation, Licht explains. Rhizobium bacteria establish a symbiotic relationship early in the season, living in nodules on roots and pulling nitrogen from the air in exchange for a place to live.

Does starter nitrogen fertilizer help soybeans? No, it can hurt, Licht says. Nitrate-N applied as fertilizer can be antagonistic, keeping nodules from forming or reducing the efficiency of nitrogen fixation by nodules. Soybean plants need more nitrogen during the season than bacterial fixation can supply, and they often get it from mineralization of soil organic matter. Adding nitrogen fertilizer, especially early, is not the answer.

Related:Learn soybean secrets from critical reproductive stages

What if soybeans haven’t been grown in the field recently? Consider inoculating those fields with Bradyrhizobia bacteria, available from many input suppliers. Inoculation of soybeans may also be considered if fields were fallow from ponding, Licht adds.

How can I tell if bacteria are active during the season? Dig up a few roots. Pinch a few nodules. If nodules are pink inside, bacteria are active and nitrogen fixation is occurring.

Are there soils where nodules may have problems? Nodulation doesn’t occur as well in acidic, low pH soils, Licht says. That is why a routine soil sampling program is recommended. Apply lime accordingly, based upon soil test results. This could require variable-rate liming in fields where pH levels vary widely.

Are all nutrients equally mobile inside soybean plants? No. Licht notes that nitrogen is most mobile, while calcium is the most immobile nutrient. See the graphs below, which illustrate how nitrogen is redistributed inside the plant during the season, while calcium moves very little. Phosphorus and sulfur are similar to nitrogen in mobility. Potassium is redistributed from vegetative plant parts, including leaves, but not from pods.

A graph showing Nitrogen accumulation in soybeans

A graph showing Potassium accumulation in soybeans

A graph showing Calcium accumulation in soybeans

How mobile are micronutrients? Zinc and copper are redistributed within the plant as the season progresses, but not like nitrogen moves, Licht says. Manganese, magnesium, iron, boron and molybdenum are relatively immobile, but not as much as calcium.

Why is redistribution of nutrients inside soybean plants important? It’s a primary source of newer growing plant parts, especially later in the season, Licht explains. Redistribution of nitrogen, phosphorus and sulfur provides the bulk of the nutrients for seed growth. Meanwhile, leaves, petioles, stems and pods become severely depleted in these elements by the end of the season.

When is iron deficiency chlorosis most likely to appear? It shows up most often on calcareous soils with pH above 7.4. Cool, wet conditions early in the growing season may also make it more likely to appear. The best management tool is selection of varieties that tolerate this condition, Licht says. Adding iron as an early foliar application may help, but not as much as selecting tolerant varieties.

When will calcium concentration be highest in soybean leaves? Concentrations of calcium rise late in the season as other nutrients move out to grow seeds and calcium stays behind.

Commentary:
From the mouths of babes — soybean knowledge

The best part about judging 4-H crops at county fairs is interacting with young 4-H’ers. I have judged crop exhibits at the Franklin County Fair in Brookville, Ind., long enough that I have imparted some basic knowledge to 4-H’ers who come back each year. Anyone past his or her first year in the soybean project knows what the big bumps on soybean roots are and what they do: nodules that fix nitrogen.

But at this year’s fair, one teenager surprised even me. More than a dozen exhibits of 10 soybean plants each waited to be judged, and all were of excellent quality. So, I was listening closely to each young person’s answer to help separate them. One young lady marched up to her sample of soybeans when it was her turn and began talking before I could even speak.

“You won’t find as many nodules on roots of my soybeans as on other plants,” she began.

“Oh, you know what nodules are?” I asked.

“Sure, bacteria live inside, and they pull nitrogen out of the air that plants can use, and that’s why Dad doesn’t fertilize soybeans with nitrogen like corn,” she explained.

“That’s right. So, why aren’t there as many nodules on your plants? They look healthy otherwise,” I responded.

“That’s because this field was fertilized with manure,” she said. “Dad said the manure has lots of nitrogen in it that went into the soil. Since there is already nitrogen there, bacteria aren’t encouraged to take up residence in the roots. Plants are already getting nitrogen.”

Whoa! That was from a middle schooler, folks. Who says kids only pay attention to cellphones and music videos these days?

About the Author

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.

Subscribe to receive top agriculture news
Be informed daily with these free e-newsletters

You May Also Like