Soybean iron deficiency chlorosis
Soybeans in North Dakota always look green as they emerge. Iron in soybean is mobile in the plant until the first trifoliate leaf emerges. At that time, for some mysterious reason iron becomes immobile in the plant and has to be taken up fresh for each increment of added tissue. Therefore, soybean that has been green up until now will suddenly become chlorotic if soil conditions are favorable for its development once the first trifoliate leaf emerges. Soil conditions that are essential for IDC development are carbonates. Soils that have pH higher than 7 normally contain measurable carbonates. Soils with pH less than 7 do not have carbonates and IDC is not possible. Yellowing of soybean in acid soils is caused by something other than iron. This year, that yellowing could be caused by waterlogged soils, loss of S or failure of inoculation in first-year soybean fields. Manganese deficiency is unknown in the field in North Dakota. Be advised that manganese application to IDC fields can intensify IDC by competing with iron for uptake.
IDC is minimized by choosing a tolerant variety, using an ortho-ortho-EDDHA fertilizer at planting, such as Soygreen™, planting in wider row spacings, using a cover crop at the time of soybean seeding, and not applying N to the soybeans or at least choosing fields with lower residual soil nitrate levels. IDC is made more severe when soils are wet (a current problem), cool (not a problem anymore), soluble salts are high, or residual nitrate is high for natural reasons or because N was applied with another crop in mind and the field switched to soybean for planting date reasons. Some growers might have used an EDDHA product at seeding and still have intense IDC symptoms. Manufacturers are not required to put the type of chelate on their label. According to Dr. Goos in our department, many EDDHA fertilizers he has tested have lower percent ortho-ortho-EDDHA in their formulation compared to Soygreen. It is the ortho-ortho-EDDHA that is particularly effective against IDC, not ortho-para-EDDHA. Many formulations of EDDHA have significant and often dominant ortho-para-EDDHA instead of mostly ortho-ortho.
Herbicide application will intensify the IDC symptoms briefly, but it is not the root cause of the problem. If IDC is seen in the field, there is no spray that can help. The grower needs to ride it out and soil drying will eventually reduce the problem in most areas of the field if a tolerant variety was grown.