MSU Extension

Nitrogen

Biological nitrogen fixation plus the nitrogen mineralized from soil organic matter provides sufficient nitrogen for optimum soybean yields in most situations. Therefore, Michigan State University Extension does not recommend applying nitrogen fertilizer to soybeans. However, a profitable response to nitrogen may occur in very high yield environments (greater than 70 bushels per acre) and in irrigated production on soils having low organic matter levels if no other yield-limiting factors are present.

Researchers at Kansas State University showed that a single application of 20 lbs/ac of actual nitrogen at R3 significantly increased soybean yields in high-yield, irrigated environments. However, on-farm research evaluating this practice conducted in Michigan in 2011 and 2012 did not show an advantage to supplemental nitrogen.

Phosphorus

Soybean yield responses to applied phosphorus have not been demonstrated and are not expected when Bray P1 phosphorus levels exceed 20 ppm.

Potassium

MSU Extension recommends maintaining soil test potassium (K) levels between 0 and 30 ppm above the critical K levels to maximize soybean yields. The critical level is calculated by multiplying the cation exchange capacity (CEC) by 2.5 and adding 75. For example, the critical K level for a soil having a CEC of 10 meq/100gm is 100 ppm [(10 x 2.5) + 75].

Broadcast applications have been shown to perform as well as or better than near seed placement at planting. To avoid leaching losses, potassium fertilizer should be applied in the spring (two weeks prior to planting) and not in the fall on coarse-textured soils having CECs less than 6 meq/100gm or organic soils.

Sulfur

Due to declining atmospheric deposition, supplemental sulfur may be required to maximize soybean yields in Michigan. This is especially true in coarse-textured soils low in organic matter. Ammonium sulfate and gypsum are the recommended sources.

Sulfur is required early in the season so the fertilizer should be applied prior to planting. Twenty five lbs/ac of actual sulfur is sufficient to maximize soybean yields. Do not exceed this rate when using ammonium sulfate as the additional nitrogen may inhibit nodulation and biological nitrogen fixation.

Boron

Soybeans are classified as having a low probability of responding to applied boron even on soils having low boron soil test levels. Despite the low probability of a response, university research trials have shown modest yield responses to applied boron in the absence of deficiency symptoms (1.4 bushels per acre).

Boron is most likely to be deficient in coarse-textured soils, organic soils and lake bed soils having high pH levels. Broadcasting 1 lb/ac of actual boron blended with potassium fertilizer two weeks prior to planting is the recommended application method. Boron can also be applied to soybean foliage during early bloom at a rate of 0.25 lbs/ac. Broadcast rates should not exceed 2 lbs/ac and foliar applications should not exceed 0.5 lbs/ac to prevent crop injury.

Manganese

Manganese deficiency is the most common nutrient deficiency seen in Michigan soybeans. Deficiency symptoms are likely on muck or dark-colored sands with pH above 5.8 and lakebed or out wash soils having pH levels above 6.5. Since increasing the available manganese levels in the soil is difficult, deficiency symptoms will reoccur in the same areas each year that soybeans are grown.

Broadcast applications are not recommended due to high soil fixation. Band application of chelated manganese is also not recommended due to the high fertilizer costs. Foliar application of 1 to 2 lbs/ac of actual manganese is the most economical and effective method for correcting manganese deficiency. Additional information on identifying and correcting manganese deficiency with foliar fertilizers is available at “Identifying and correcting manganese deficiency in soybean.”

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 MSU Extension and the Michigan Soybean Checkoff program.