Amy Shober, Extension Nutrient Management and Environmental Quality Specialist; ashober@udel.edu and Jarrod O. Miller, Extension Agronomist, jarrod@udel.edu
Micronutrient deficiencies, particularly manganese (Mn), are often overlooked, but can significantly reduce soybean yields. In Delaware, most reports of Mn deficiency in soybean come from western Kent County. However, many sandy soils in Delaware are susceptible to Mn deficiency if pH is not managed properly, which is most likely to occur when soil pH is higher than 6.0.
Preplant soil tests offer a quick method to screen soils for potential Mn deficiencies, and many soil testing laboratories offer micronutrient testing as a routine analysis. If micronutrients are not part of the routine test, consider requesting this analysis as an “add-on” if you have experienced soybean Mn deficiency in the past.
Some soil test reports may include soil test Mn values and include a visual interpretation of those results (e.g., low, medium, optimum, excessive). However, Mn availability in soils is pH dependent, with availability decreasing as soil pH increases. The Mn availability index (MnAI) calculation accounts for the effect of pH on soil Mn availability and should be calculated using the equation:
MnAI = 101.7 – (15.2 × soil pH) + (2.11 × M3-Mn)
where MnAI is the Mn availability index, soil pH is measured in water (1:1 V:V), and M3-Mn is Mehlich 3 soil test Mn in lb/A. (Note: To convert soil test Mn results from ppm to lb/A multiply by 2; do not use “buffer” pH for this calculation). Interpretation of the MnAI is crop specific. Manganese deficiency in soybean is likely when the MnAI is less than 25, but unlikely when the MnAI is above 35. When MnAI is between 25 and 35, University of Delaware recommends that crops be monitored for Mn deficiencies, especially if liming was recommended.
Manganese is not mobile in plant tissue, which means that deficiency symptoms will appear in new growth because the plant cannot move Mn from old growth to new growth. Interveinal chlorosis or yellowing of the plant tissue is the most common visual symptom of Mn deficiency in soybean (Figure 1). However, because interveinal chlorosis is a common nutrient deficiency symptom, visual deficiencies should be confirmed with a tissue test. The critical level for tissue Mn concentrations at the soybean R2 stage is 17 ppm. Keep in mind that visual deficiencies indicate a more severe problem and yield reductions may be occurring in fields without obvious symptoms. Regular tissue testing is the only method that will reveal if “hidden” hunger is occurring.
Figure 1. Interveinal chlorosis in soybean appears on the new growth due to Mn deficiency.
Broadcast soil applications of 20-30 lb Mn/A can be applied to prevent soybean Mn deficiency if the MnAI is <25. Foliar applications [1.0 to 2.0 lb/ac elemental Mn as Mn sulfate or Mn oxide; 0.5 to 1.0 lb/A elemental Mn as chelated Mn (Mn-EDTA)] can be used to correct soybean Mn deficiency in season if confirmed by a tissue test. Foliar applications should be applied only when adequate growth is present to aid absorption.
University of Delaware researchers established replicated in-field strip trials in 2019 and 2020 and small-plots at the University of Delaware Warrington Irrigation Farm in 2020 to evaluate foliar application of Mn to soybean at two rates (0 and 1.5 lb/ac Mn) on crop yield and tissue concentrations. Despite soil test reports that suggested “low” to “optimum” concentrations of soil test Mn, all sites had a calculated MnAI that was >35. We reported no significant yield or tissue Mn response to foliar Mn application at any site. As such, we do not recommend that farmers apply foliar Mn unless soil MnAI is below 25 or if Mn deficiency was observed and documented previously by a tissue test. Additional research is needed on soils with suspected Mn deficiency (MnAI <25) to determine the economic value of foliar Mn applications to soybean.