Mark Reiter, Extension Soils Specialist, and David Holshouser, Extension Agronomist
Manganese (Mn) deficiencies are common in Virginia soybean, but these deficiencies are not necessarily due to low Mn levels in the soil. Instead, like many micronutrients (nutrients that are needed by the plant in small amounts), Mn availability to the soybean crop is directly related to soil pH.
When pH levels reach 6.5 or above, Mn deficiencies will likely appear, especially on sandy soils. However, some soils with a pH of 6.2 and lower can show deficiencies if soil Mn is low. Generally, Mn deficiencies are more common on our sandier soils as pH changes more rapidly and sandy soils typically have a lower Mn concentration.
Manganese deficiencies will also reveal themselves with dry soil conditions, especially on tilled soils. This is because Mn becomes less available under oxidizing conditions. Oxidizing conditions occur under dry environments where there is more oxygen and less water available in the soil pores. In this situation, Mn oxides form (basically a rock) and Mn oxides are not available for plant uptake. In places with more soil compaction, such as wheel tracks, or under wetter conditions (more pore space is occupied by water), Mn oxides will be reduced to Mn2+, the form of plant uptake. This is why we often see Mn deficiencies throughout a field but not in wheel tracks where the soil is more firm. Firmer soils don’t have as much pore space; therefore, they have less oxygen to form Mn oxides. Shown below is an example of this.Finally note that other problems can cause look-alike symptoms similar to Mn deficiencies. In particular, inter-veinal yellowing is a common symptom of soybean cyst or other nematode damage. Therefore, it may be prudent to further investigate the problem, especially the root system.
Use the following guidelines for Mn applications:
Scout your fields. Mn deficiencies may or may not materialize. The only sure way to determine a deficiency is to observe the deficiency symptoms through visual observation or tissue tests. The characteristic visual symptom is yellowing between the veins on the new leaves. Mn is an immobile nutrient. Therefore, it will not move out of older leaves to the new leaves. Symptoms will appear when the plant can no longer extract sufficient amounts of the nutrient from the soil.
Take a tissue sample. If Mn deficiencies are suspected due to high pH and/or a field history of Mn deficiencies, but no symptoms have yet appeared, you should consider taking a tissue sample. Tissue samples can reveal deficiencies before symptoms appear (hidden hunger). We suggest a tissue test if lime, lime stabilized biosolids, or an ash product was recently applied.
Manganese application. To overcome a deficiency, apply ¾ lb. chelated Mn (elemental basis) or 1 lb. inorganic Mn (elemental basis) per acre to foliage upon appearance of symptoms and prior to flowering. More than one application may be required to correct a severe deficiency.
Don’t use low rates to correct a deficiency. Note that many Mn products recommend applying lower rates of Mn. However, the label usually states that these are maintenance rates. Once a deficiency occurs, these lower rates will not correct the deficiency and the rates stated above will be needed.
Split Mn application on deficiency-prone soils. An alternate method of application can be used before a deficiency is evident on soils that commonly show a deficiency, especially on soils that have a high pH (above 6.8 or so). A lower rate (~ ½ of that listed above) can be combined with another scheduled application, such as a postemergence herbicide or insecticide. This may be a sufficient rate to prevent a deficiency from occurring. But, continue to scout the field and take future corrective measures if visual deficiencies appear. If a visual symptom appears, you need to use the full rate. I will remind you that this is a preventative treatment. A deficiency may not occur. Furthermore, these are only maintenance rates and another application will likely be needed if the field is truly deficient.
Use EDTA chelated Mn formulations when mixing with glyphosate. Be reminded that some Mn formulations in combination with glyphosate herbicide (Roundup, Touchdown, many generics, etc.) will result in reduced weed control of certain weeds. Other herbicides have not shown to interact. If including Mn with glyphosate, use the EDTA chelated formulation as it has shown not to interact.
Don’t spray if you don’t need it. Mn can be toxic to soybean. Spraying greater than recommended rates or spraying as a preventative spray when soil pH is relatively low (5.7-5.9) could lead to toxicity problems