Risks and benefits of applying boron fertilizer to soybeans

Soybean producers should understand the risks and benefits of applying boron fertilizers to soybeans.

Soybean yield responses to applied boron fertilizer have been variable in previous research trials. This is not surprising given that soybeans are classified as having a low probability of responding to boron fertilizer even at low soil test boron levels. Boron is more likely to be deficient in coarse-textured and organic soils due to leaching losses. Boron availability also declines in lake bed soils as soil pH increases from 6.5 to 8.0. Yield responses to applied boron are more likely to occur under these conditions.

Both granular and liquid boron fertilizers are available. A pre-plant broadcast application of granular boron blended with potash is recommended on potentially responsive sites. However, liquid boron fertilizer can be applied with post-emergence herbicide applications. To avoid leaf burn, foliar boron application rates should be less than 0.5 pounds per acre and do not spray when temperatures are high and the crop is under moisture stress. Boron fertilizers applied near the seed have been shown to reduce germination. Therefore, band applications are not recommended and broadcast applications should be made one to two weeks prior to planting. Boron toxicity can be further reduced by applying less than 2 pounds of actual boron per acre to the soil or less than 0.5 pounds of boron per acre to the foliage. Boron fertilizer toxicity is more likely to occur under acidic soil conditions.

Despite the low probability for soybeans to respond to boron fertilizer, yield increases have been reported in some university trials. Research conducted at 29 sites in Wisconsin, Ohio, Illinois and Missouri showed an average yield increase of 1.4 bushels per acre from a single foliar application of boron. The boron was applied at 0.25 pounds per acre at the R1 growth stage. Higher application rates of foliar boron and soil applied boron did not improve soybean yields at these sites. The soils at the research sites were not considered deficient in boron.

Boron also increased soybean yields at three out of four research locations in Arkansas. At one of the sites, boron increased soybean yields by more than 100 percent while yield increases at the other two sites ranged from 4 to 15 percent. Three of the four sites had shown boron deficiency symptoms in previous soybean crops. The researchers found that 0.25 to 1 pound of actual boron per acre was adequate to maximize soybean yields. They also found that boron deficiency symptoms and subsequent yield response were more closely predicted by plant tissue analysis than by soil testing. Plant tissue boron concentrations of 0.2 to 0.5 ppm are considered sufficient.

Boron fertilizer effects on soybean yields have been evaluated in on-farm research trials conducted in Michigan in 2005, 2011 and 2012. A foliar application of 0.25 pounds per acre of actual boron applied at R1 was compared to an untreated control at one site in 2005, four sites in 2011 and one site in 2012. The boron fertilizer did not affect soybean yields in any of these trials. A pre-plant broadcast application of a granular boron fertilizer was compared to an untreated control at one location in 2012. While the 2.2 bushels per acre increase at this site was not statistically significant, it is close enough to warrant further investigation, especially on potentially responsive sites.

This article was produced by the SMaRT project (Soybean Management and Research Technology). The SMaRT project was developed to help Michigan producers increase soybean yields and farm profitability. SMaRT is a partnership between Michigan State University Extension and the Michigan Soybean Checkoff program.

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