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Iron Deficiency Chlorosis (06/24/21)

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Iron deficiency chlorosis (IDC) is starting to show in soybean fields in the Red River Valley. There is usually enough iron (Fe) available in the soil; however, young soybean plants may not be able to take up enough Fe. Iron chlorosis can appear if the soil is calcareous and there is sufficient moisture to dissolve carbonates to produce bicarbonate to neutralize root acids in the soil. Apparently, there is enough moisture in many fields to produce bicarbonate and initiate IDC. Stressed plants usually exacerbate IDC symptoms. Elevated soil salts levels often result in greater plant stress and more severe IDC symptoms. With excess moisture, cool temperatures, and/or high soil nitrate content, IDC symptoms can be more severe. An application of some post herbicides may also briefly stress plants and move them into a state of increased IDC.

The cotyledon and unifoliate leaves of soybeans emerge green because Fe is mobile in the plant until the trifoliolate leaves emerge. As the trifoliolate leaves start to develop, the plant’s physiology changes and Fe becomes immobile, and the essential Fe needs to be taken up by the soybean roots. This is when chlorosis often starts to appear if soil and environmental conditions are favorable (Photo 1) for IDC expression. The IDC symptoms appear as yellowing leaf tissue between the veins while the veins may remain green. In a severely affected plant, the tissue will become brown and tissue may die. Plants with chlorosis are often stunted and growth and development are slow compared to healthy plants. Soybean plants may grow out of the chlorosis and become green again; however, yields are usually reduced due to IDC.

Photo 1. Soybean plants with iron deficiency chlorosis (IDC) near Fargo, ND, mid-June 2021

The most important management practice to reduce IDC, in IDC prone soils (pH greater than 7 and free carbonates present), is selecting chlorosis tolerant soybean varieties. At NDSU, scientists use a rating of 1 to 5 to score the IDC symptoms, where 1 is green, 3 is yellow, and 5 is dead tissue (Photo 2).

Photo 2. Scale used for IDC scoring of soybeans

Soybean varieties have genetic differences for tolerance and susceptibility to IDC symptoms. No soybean variety is immune to the chlorosis, but large differences in yellowing, subsequent plant stunting, and reduced yield occur between the most tolerant and most susceptible varieties (see graph 1). However, when IDC is anticipated in a field, producers should select varieties with tolerance to IDC and high yield potential. For instance in graph 1, varieties with a similar IDC score of 2.5 yielded between 17 and 32 bushel per acre.

Graph 1. IDC score and yeild, NDSU variety trial with 40 different varieties

Other management strategies in addition to variety selection to reduce IDC include: proper field selection, using an ortho-ortho-EDDHA chelate Fe treatment with a proven track record of performance at seeding, an increase in seeding rate, seeding in wider rows with the same seeding rate (plants closer together within the row) if weed management will allow it, managing the nitrate content in the field using a companion crop at the beginning of the season, and managing the crop to avoid additional stress conditions. It is important to note that since Fe is not mobile in soybeans after trifoliate leaves emerge, application of foliar Fe fertilizers are not useful nor economically practical.

Hans Kandel
Extension Agronomist Broadleaf Crops

Dave Franzen
Extension Soil Specialist