Interest and concern about micronutrient needs of soybean is common among farmers and agronomists in southeast Nebraska (Figure 1) due to higher yields than in previous decades and the increased availability of micronutrient fertilizers in both liquid and dry formulations. Nine micronutrients are needed in small amounts by soybeans. Micronutrients include boron (B), chlorine (Cl), cobalt (Co) copper (Cu), iron (Fe), manganese (Mn), molybdenum (Mo), nickel (Ni), and zinc (Zn). We bring to you an extended series of CropWatch articles on micronutrient fertility and plant nutrition for soybean production in southeast Nebraska (Figure 1). Last week we introduced this the series, so if you missed it, we encourage you to go back and read it: New Series on Soybean Micronutrient Management in Southeast Nebraska. If you haven’t already, please take the quick 2-question survey and share with us what micronutrients you are most concerned about on your farm for soybeans.

Figure 1. Southeast Nebraska USDA Crop Reporting District Take the Micronutrient Survey

Soybean Boron Deficiencies and Toxicities in Southeast Nebraska

In soybeans, boron is needed for cell walls during cellular expansion and normal development of nitrogen-fixing root nodules. Boron deficiencies show up at growing points because boron is not easily moved from old to new tissue. Deficiency symptoms consist of stunting, swollen nodes, and the 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, including scorching/necrosis on the leaf edges (Figure 2).

Potential micronutrient issues are regional in nature. Boron deficiency is extremely rare in southeast Nebraska and has not been observed by University of Nebraska-Lincoln Extension to date. The closest documented boron deficiency has been with alfalfa in central Nebraska on sandy soils, but not soybeans. In boron sensitive crops like alfalfa, potassium (K) fertilization can displace calcium on the soil exchange sites and high calcium in solution can interfere with boron absorption. Northeast Arkansas is the closest area with regular soybean boron deficiencies issues, especially when soybeans follow rice.

Figure 2. Boron toxicity in soybeans showing symptoms of scorching or necrosis on the leaf edges from over fertilization (Photo by Nathan Mueller)

Soil Boron Availability to Soybeans

Soil availability of boron is greatest when the soil pH is between 5.0 and 7.0. Boron in soil solution exists as a non-ionized molecule called undissociated boric acid (H3BO3), 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. Since organic matter is a large pool of potentially available boron, drought can decrease availability due to slower decomposition and less movement of boron to roots in dry soil. Boron deficiencies in soybeans are most likely to occur on low organic matter sandy soils. Low organic matter sandy soils are limited or make up very few acres in southeast Nebraska, however areas and soil series names include:

• Sandy alluvial soils along the Missouri River called the Sarpy soil series
• Sandy glacial soils (glacial outwash deposits reworked by wind) in Gage, Johnson, and Otoe counties called the Dickinson and Filley soil series
• Sandy and gravelly soils in Jefferson, Thayer, and Nuckolls counties called the Meadin and Jansen soil series
• Sandy alluvial soils in Nuckolls County along the Republican River called the Inavale, McCook, and Wann soil series

Soil and Plant Tissue Testing

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 about how to conduct soybean plant tissue sampling by watching a video created by Nathan Mueller a few years ago. The boron sufficiency range is 25 to 60 ppm. If in the sufficiency range, boron is not likely a yield-limiting.

Boron Fertilizer Suggestions

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. Boron concentrations in open lot beef manure is very low and not a major source of boron. However, groundwater irrigated soybean acres are receiving some boron based on past work at UNL. For example in Saline County, 0.2 lbs of boron would be applied with 6-inches of applied groundwater based on water analysis from a well west of Wilber. Read a recent CropWatch article and view the maps of nutrient contributions by county: Crediting Essential Nutrients Applied in Irrigation Water. If you are still concerned about boron, we encourage you to conduct your own on-farm research through the UNL On-Farm Research Network by contacting us.

Other resources:

• Micronutrients for Soybean Production in the North Central Region
• Symptoms Identification Key for Nutrient Deficiencies in Soybeans
• Nutrient Management for Agronomic Crops in Nebraska (pages 43-50 and 118-119)
• NebGuide: Micronutrient Management in Nebraska
• Nebraska Soybean and Corn Pocket Field Guide – 2019 Edition – (pages 263-269) Visit extension offices in Saline, Jefferson, Gage or Clay counties for a free hard copy