Welcome to part two of the soybean micronutrients series. Yesterday I introduced the series and set the stage for discussing boron (B) this week, our first micronutrient of nine. 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. I hope you had time to glance over a few of the extension publications I mentioned two weeks ago including Nutrient Management for Agronomic Crops in Nebraska or Micronutrient Management in Nebraska at extensionpubs.unl.edu, or pick up a free hard copy of the 2019 Nebraska Soybean and Corn Pocket Guide at the Extension office.
In soybeans, boron helps form cell walls during expansion and helps with normal development of root nodules for nitrogen fixation. Boron does not easily move 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. Toxicity symptoms include scorching and necrosis on the leaf edges.
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, which is unique among micronutrients. Control of soil boron availability is 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, I recommend using plant tissue nutrient analysis to determine boron sufficiency. Soybean boron sufficiency is best determine during the beginning to full bloom stage, typically in early July. Collect the uppermost fully expanded trifoliolate without the petiole from 30 random plants and send them to the lab in a paper bag. Learn more how to soybean plant tissue sampling by watching my video on YouTube called “How to Take a Soybean Leaf Sample.” The boron sufficiency range is 25 to 60 ppm. If in the sufficiency range, boron is not likely a 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, application of low rates of boron occur 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. Feel free to email me at firstname.lastname@example.org or call me at 402-821-1722 to submit a question about micronutrients. I looked forward to writing about chloride in my next biweekly column. Know your crop, know your tech, know your bottom line at croptechcafe.org.