Against the dark green backdrop of healthy soybeans, you can sometimes see intense yellow patches of soybean leaves in pockets of some fields. More often than not, this is a visible symptom of manganese (MN) deficiency in the beans.
MN deficiency is most likely to occur in soils where the pH is high and the moisture level is low, and in soils that are naturally low in MN. Locally, I see MN deficiency symptoms most often on high organic matter soils, such as the muck lands that scattered in several regions in the community.
Research at Purdue has shown that soybeans low in MN can lose as much as 12 bushels per acre. Tissue tests of deficient plants are often lower than 10-12 parts per million of MN. The researchers have learned that soybean plants above 20 ppm of MN generally produce well, but they have seen responses when MN levels increase to 30-35 ppm.
On mineral soils with a high pH, lowering the pH to 6.0 to 6.6 range allows the plant to “find” the MN that is the field but unavailable to the plant. Lowering the pH is done with applications of sulfur.
I have seen fields with mineral soils with pHs in the 5.0-5.5 range that developed patches of MN deficiency after an application of lime. The sudden appearance of MN deficiencies in patches throughout the field is because the lime was applied uniformly, not taking into account some pockets of soil are already high in pH. The addition of lime in those spots pushes the pH over 6.7, and triggers MN deficiency. Applying lime with variable rate technology might help solve some of those issues.
Soybean farmers should also keep a close eye on their fields for visible symptoms of manganese deficiency following a glyphosate application to make sure the soybean leaves maintain manganese concentrations above 20 ppm.
If a manganese deficiency is visible on glyphosate-resistant beans, Purdue recommends a foliar application seven to 10 days after the glyphosate application. It is not advisable to mix MN with glyphosate because the performance of both is compromised.
Where pH has been adjusted and MN is still deficient, two or three foliar applications of MN can be made during the season. Soil applications of MN have not proven to show as much yield response as the foliar treatments.
Adjusting the pH of high organic-matter soils, like the mucks I wrote about earlier, is a more complicated issue. The pH of most of these organic soils often gravitates to 4.5 to 5.5, in other words, very acidic. Lots of odd things happen to the availability of plant nutrients if you try to adjust muck pH above 6.5 or higher. Generally, most growers try to maintain the pH of muck at somewhere around 5.2–5.5 and use foliar applications of MN to relieve any deficiency that might show up.