Yellow Soybeans - Is It Mn Deficiency???

Symptoms

Symptoms of Mn deficiency are interveinal chlorosis (yellowing) on the newest trifoliates, although older trifoliates often show the symptoms as well. Mn is immobile in the plant, so symptoms will generally appear on the younger leaves (Figure 1).

Figure 1- Mn Deficiency Interveinal Chlorosis

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Simple Test

If it’s unclear if Mn deficiency is the problem, take a hand spray bottle (ie:windex type) of Mn in solution, and hand spray 1 metre of row. Flag this section. If Mn is the problem, this section of row will turn dark-green in 2-3 days. If the area does not turn dark-green further analysis is necessary to determine the culprit. 

Factors Influencing Mn Deficiency

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. That’s why Mn deficiency is not generally found in soils with a pH below 6.2. Deficiencies can also appear on eroded knolls where the pH is higher than the rest of the field. 

The deficiency is most common on poorly-drained soils, especially clays and silt loams. High organic matter also ties up Mn. Symptoms may occur in hollows where organic matter is higher. 

Manganese is less soluble in well-aerated soils. That’s why in compacted areas (wheel tracks) a dark-green colour may remain while the rest of the field becomes symptomatic (Figure 2).

Figure 2 - Mn Deficiency In Compacted Wheel Tracks 

An over-riding factor is that roots must reach manganese to absorb it. Wet soils, extremely dry soils, cool weather, soil compaction, root diseases, nitrogen deficiency and herbicide damage can limit root growth causing Mn symptoms. 

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Yield Losses

Correct the deficiency once symptoms are evident. A delay of 2 weeks may decrease yields by 10%. A 6-week delay may result in 30% yield loss. However, yield benefits will only be evident in those parts of the field that actually show symptoms. There is no benefit to spraying a field that shows no symptoms. 

Remedy

The general recommendation is to spray the foliage with 2 kg/ha of actual manganese from manganese sulphate (8 kg of manganese sulphate/ha) in 200 L of water. A "spreader-sticker" in the spray is recommended. If the deficiency is severe, a second spray may be beneficial. Chelated sources of Mn are usually applied at lower rates. Check the recommendations for the specific product. Several products are now available. If you have Mn deficiency and would like to be involved in a spray trial please contact Horst Bohner through Ag Info.

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