Welcome to part two of the micronutrients in soybeans column series. Last week I introduced the series and set the stage for discussing boron (B) this week, our first micronutrient of nine that I plan to cover. I will cover how boron is essential to soybeans, known deficiencies and toxicities in the region, factors of boron availability in soil, soil and plant tissue testing, and fertilizer recommendations. Hopefully you had time to glance over a few of the extension publications I mentioned last week including our Extension Circular on Nutrient Management for Agronomic Crops in Nebraska and our NebGuide on Micronutrient Management in Nebraska.
In soybeans, boron is needed for cell walls during expansion and for normal development of root nodules for nitrogen fixation. Boron is not easily moved from old to new tissue, so boron deficiencies show up at the growing points. Deficiency symptoms consist of stunting, swollen nodes, and death of the growing points. Older leaves may appear thick, dark green, leathery and cupped downward, and delayed leaf loss or senescence in the fall. Overall, soybeans are rather insensitive to boron deficiency, but very sensitive to toxicity (scorching/necrosis on the leaf edges as seen in Figure 1 above).
Soil availability of boron is greatest when the soil pH is between 5.0 and 7.0. Boron in soil solution exist as a non-ionized molecule as B(OH)3, which is unique among micronutrients. Soil boron availability is controlled by adsorption/desorption on surfaces of aluminum and iron oxides, clay minerals, calcium carbonate, and organic matter. Deficiencies most likely to occur on low organic matter sandy soils. Boron deficiencies are rare and only found in alfalfa in central and sugar beets in north central Nebraska on sandy soils, but never soybeans. Northeast Arkansas is the closest area with soybean boron deficiencies issues.
Soil sampling and analysis does a poor job at predicting the need for boron fertilization and poorly correlates with soybean boron uptake. As a result, it is recommended to use plant tissue nutrient analysis to determine boron sufficiency. Soybean boron sufficiency is best determined during the beginning to full bloom stage, typically in early July. The uppermost fully expanded trifoliolate without the petiole from 30 random plants should be sent to the lab in a paper bag. Learn more how to sample soybean plant tissue by watching a video below. The boron sufficiency range is 25 to 60 ppm. If in the sufficiency range, boron is not likely yield limiting. The need to apply boron fertilizer to soybeans in southeast Nebraska is extremely low based on our current knowledge and limited acres of low organic matter sandy soils. Additionally, due to boron in local groundwater, low rates of boron are being applied under irrigation. If you are still concerned about boron, I encourage you to conduct your own on-farm research through the UNL On-Farm Research Network by contacting me.
Learn more in a recent article: Plant Nutrient Analysis: Do your soybeans have the right stuff?
Feel free to email () or call me (402-821-1722) to submit a question related to boron or chloride. I look forward to the ongoing news column series on micronutrients in soybean production including next week’s on chloride. You can share or read this news column online through my local agronomic website for Saline, Jefferson, and Gage counties at croptechcafe.org. Know your crop, know your tech, know your bottom line.
