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.
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.
NOTE MOBILITY: Nitrogen is very mobile inside the soybean plant and accumulates in the beans (seeds) near harvest.
POTASSIUM MOVES: Potassium is not as mobile as nitrogen, but it also moves within the plant, accumulating in seeds.
IN LEAVES: By the end of the season, most calcium is found in fallen leaves and petioles. Calcium is the most immobile nutrient.
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?
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