Manganese Deficiency

Manganese deficiency is becoming evident in some fields. Soybeans naturally go through a period when leaves are light green before the nodules start to supply adequate nitrogen and a dark green colour returns. If proper nodulation and sufficient nutrients are present soybeans will come out of this phase relatively quickly. However, yellow leaves with dark veins are an indicator of Mn deficiency.

Symptoms of Mn deficiency are interveinal chlorosis (yellowing) on the newest trifoliates. The level of chlorosis can vary from a slightly paler green colour in the interveinal tissue, through yellow, to almost white, but the common factor is that the veins remain dark green. Mn is immobile in the plant, so symptoms will generally appear on the younger leaves, but older trifoliates may show the symptoms as well if the deficiency has been present for long enough. In some cases, the plant will outgrow the deficiency as the root system expands, or as Mn mineralizes from soil organic matter, so the upper leaves are normal while the symptoms persist on leaves lower in the canopy.

One of the most significant factors affecting the availability of Mn is the soil pH. As soil pH increases, less Mn is available to the plant. This is why Mn deficiency will most commonly appear on eroded knolls where the pH is higher than the rest of the field. The second factor in Mn availability is soil aeration, with lower availability in well aerated soils. This results in Mn deficiency showing up on poorly-drained soils that have been tile drained. It also explains the very diagnostic pattern in the field of dark green plants in the tire tracks, while the areas between the tracks show interveinal chlorosis. High organic matter (peat and muck soils) also ties up Mn, resulting in plant deficiency in depressional areas of the field.

Correcting true manganese deficiency with a foliar application of the micronutrient is recommended. Soil applied Mn has generally not been effective in Ontario, because the problem is unavailability of the Mn in the soil, rather than an actual lack of Mn. Beans will show a profitable response to manganese in the parts of the field where the deficiency is evident.

Figure 1. Typical Manganese Symptoms in Soybeans

There is no benefit to applying manganese to beans that are not showing a deficiency. Various sources of foliar Mn are available, but the most effective and economical is manganese sulphate. Spray the foliage with 2 kg of actual manganese/ha from manganese sulphate (8 kg of manganese sulphate/ha) in at least 200 L/ha of water. A "spreader-sticker" in the spray is necessary, both to ensure good coverage of the leaf and to assist in moving the manganese through the cuticle of the leaf. If the deficiency is severe, a second spray may be needed. Chelated sources of Mn are also available, but at a higher cost. Opinions vary as to the effectiveness of the chelated products. Ontario research showed they were equally effective to manganese sulphate, when applied at the same rate of actual Mn, while Michigan trials have suggested they could be used at half the rate of manganese sulphate and provide equal results. Check the recommendations for the specific product.

Manganese deficiency in Roundup Ready™ Soybeans

There have been reports in the literature of increased incidence of Mn deficiency symptoms when Roundup is applied to Roundup Ready soybeans. Researchers at Purdue University in Indiana have attributed this to a reduction of the effectiveness of manganese use within the plant, and perhaps to interference with manganese uptake from the soil. A more recent study at the University of Illinois documented that the amount of "flash" on the soybean leaves increased with higher rates of glyphosate, but that foliar application of manganese had no impact on the amount of "flash", the leaf Mn content, or crop yield. It is possible that a yield impact would only be seen in sites that were approaching deficiency in manganese.

Caution must be exercised if mixing Mn with glyphosate and other herbicides. Dry flowable products show the most antagonism, while chelated versions show the least. Antagonism may result in reduced weed control. Check the recommendations for the specific product. There have also been reports in the literature that glyphosate inhibits the uptake of foliar manganese applied prior to, with, and for up to eight days after glyphosate application.