Dr. Dan Talks Agronomy

Boron Important for Soybeans

A dead terminal growing point is one of the symptoms of boron deficiency in soybeans. (Photo courtesy of Nathan Slaton, University of Arkansas System Division of Agriculture)

To me, boron (B) has always been a micronutrient that was necessary in small quantities. Historically, my belief was the soil supplied the micronutrient in ample quantities and any supplemental application created a risk of toxicity.

I recently learned that B deficiency is a risk with soybeans, and the risk of toxicity seems to be overrated.

Boron has several roles in the soybean plant: aiding cell formation, cell wall and vascular tissue formation, node number and plant height, flower development, pollen viability and ultimately pod formation and seed set.

Plants need a continuing supply of B through the season as the nutrient is not mobile in the plant. Plants can't recycle it from old tissues like they do with nitrogen, phosphorus and potassium. Once B becomes limiting, symptoms appear in young tissue and a deficiency can reduce flowering and pod set during the summer. Plants take up 70% to 75% of the boron used after R2 and through the pod set and fill stages.

Nathan Slaton, soil fertility professor at the University of Arkansas, said if boron is limited during reproduction, vegetative growth would appear normal but pod and seed set could be impeded and not even noticed. Slaton talked about the importance of boron on soybeans in a recent webinar, available here: http://ilsoyadvisor.com/…

Sandy soils, soils with low CEC and organic matter are most vulnerable to B deficiencies. A high potassium (K) soil test level can reduce boron availability. For example, a K-to-B ratio of 200-to-1 is about normal while a ratio of 2,000-to-1 can induce a boron deficiency. These conditions can happen when a lot of manure has been applied.

Slaton explained that deficiencies can occur in areas of low rainfall when root activity is restricted and in alkaline or strongly acidic soils. "The boron ion (borate is like nitrate) is mobile in soil and can be leached from the root zone during heavy rainfalls, especially in lighter sandy soils. And recent lime applications can also create a temporary B deficiency."

Slaton considers soil tests an unreliable measure of boron availability. He has seen soil tests show low boron yet sufficient tissue test levels and vice versa. "Tissue testing is the most reliable measure of B deficiency and 20 ppm is the sufficiency threshold," he said.

"Spend the $15 or $20 on a tissue test in season to know whether your soybeans are deficient or sufficient."

Timing of tissue testing is important. Pulling tissue samples at late R3 and R4 are best at predicting a response to B. However, Slaton pointed out that visual deficiency symptoms aren't seen until levels drop to 12 parts per million. By then, yields will be reduced.

Boron-deficiency symptoms first appear as growth abnormalities resulting in shortened internodes and yellowing of new leaves. Look for stunted plants with fewer flowers and pods. Slaton said one of the other symptoms is a dead terminal growing point. The symptom may be caused by calcium or B deficiency. However, if exchangeable calcium levels and calcium base saturation is high, B is probably the culprit. Lastly, soybeans that are B deficient will yellow more slowly in the fall than plants with sufficient B.

Slaton said the risk of B toxicity is rare. In the past 15 years, he has only seen one field show toxicity symptoms, which is necrosis of leaf edges. He considers 20 to 60 ppm sufficient and 60 to 100 ppm at risk for toxicity. When he saw leaf symptoms, tissue concentrations were greater than 200 ppm. He considers such high concentrations unlikely with normal soil and foliar-applied applications.

If soybeans turn yellow due to lack of nitrogen fixation, a dose of nitrogen can correct that symptom. Slaton said that's not the case for B deficiency: Once it appears, it can't be corrected. That is why he stresses prevention over correction.

Granular B can be blended with dry fertilizer and broadcast at a rate of 0.5 to 1 pound per acre. It can also be applied as a foliar in season at a rate of 0.1 to 0.3 pound per acre.

Dan Davidson can be reached at AskDrDan@dtn.com