Yellow soybeans have been spotted in fields throughout western Minnesota this year.

Several Extension staff members, including nutrient management specialist Dan Kaiser, soybean specialist Seth Naeve and educators Angie Peltier and Anthony Hanson explained the issue of yellowing, an iron deficiency chlorosis (IDC), during a recent Strategic Farming program.

IDC does not occur due to a deficiency of iron in the soil. It is caused by the inability of some plant species to extract iron from the soil in a plant available form. This results in the yellowing, or chlorosis, of leaves between the leaf veins while the veins remain dark green.

IDC is particularly a problem in soils with a high pH, and in soils with large amounts of calcium carbonate. Compaction and poor soil drainage can also affect the availability of iron, while environmental conditions influence the development of IDC.

The amount of bicarbonate in the soil correlates with IDC in soybean — the more bicarbonate in the soil, the higher the occurrence of IDC. Bicarbonates are formed in the soil typically when water-logged soils trap carbon dioxide.

There is also a linkage between high soil nitrate content and IDC. IDC issues can be greater where residual nitrates are higher in the soil. IDC may be higher in fields where drought conditions last year led to higher residual nitrate levels this year.

Planting a tolerant variety is the number one defense against IDC in soybean. Tolerant varieties have higher yield potential than less tolerant varieties where IDC is a problem, and potentially even greater yield potential when other practices that reduce IDC are used.

Applying an ortho-ortho EDDHA iron chelate source in a band directly on the soybean seed at planting has been effective at managing IDC. The greatest economic advantage can occur when areas of a field with a history of IDC symptoms are targeted with an in-furrow iron-chelate at planting, and where IDC levels have been moderate or high. Foliar applied chelates however, may only green up tissue that comes into contact with the product. New vegetative growth after treatment may still be yellow, as iron is not mobile in the plant.

A companion crop of oats with soybeans has also been shown to help reduce IDC. The oats help take excess nitrate and moisture from the soil. The oats should be terminated before they reach more than 10 inches in height, however, as competition with the soybean crop for water could be an issue when moisture is limiting.

Attempts to acidify soil may sound good in theory, but in practice, the majority of our soils have a high buffering capacity, which means they have high resistance to a pH change. It would take a large amount of elemental sulfur to result in a very small drop in pH, and pH levels would likely rise back to where they were within two to three years.