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Soybean Iron Deficiency Chlorosis (IDC) (06/14/18)

Soybeans are ‘blooming’ in the region. Not flowering, just turning yellow. My first summer at NDSU (1994) I experienced this phenomena for the first time, asked a colleague what was going on with the yellow soybeans and they said “Oh, they always do that.”

Soybean Iron Deficiency Chlorosis (IDC)

Soybeans are ‘blooming’ in the region. Not flowering, just turning yellow. My first summer at NDSU (1994) I experienced this phenomena for the first time, asked a colleague what was going on with the yellow soybeans and they said “Oh, they always do that.” But soybeans are supposed to be green all season. Yellow is bad. I have detailed the reasons for IDC in the NDSU Soybean Fertility circular SF1164, 2018 version that can be accessed as a pdf or as html.

The summary version follows:

Due to the increase in soybean acres planted this year, there are probably soybeans planted in fields that are not favorable for soybean growth. IDC cannot happen unless soil pH is greater than 7 and the soil has significant carbonates (calcium carbonate, magnesium carbonate, sodium carbonate). All soils with pH greater than 7 have at least some measureable carbonate. There are differences even within a soil type in carbonate content. A Bearden soil, common at the fringes of the Valley, may have 2% by weight carbonates in the surface 6 inches, or 20% by weight carbonates. Carbonates themselves do not influence IDC; however, when the soil is moist enough to dissolve more soluble salts like sulfate or chloride salts, carbonates (CO3-2) dissolve forming bicarbonate (HCO3-1). Bicarbonate neutralizes the acidity that soybeans produce around their roots, which is significant because the protein that soybeans exude to change insoluble iron into soluble iron only works in an acid environment (like eggs going into a frying pan- first sort of a liquid, then a solid due to heat). Enzymes are 3-D biological tools that have a function, and when the environment is unfavorable, they change shape. Think of a Phillips head screwdriver changing into a straight head.) Once the protein is inactivated, iron is no longer accessible to the soybean, and new leaves without an iron source turn yellow due to low chlorophyll production (of which iron is a key element). The greatest reason for regional IDC being different than Iowa IDC is soil salinity. Soil salinity is a huge plant stress. Whenever there is a plant stress, its ability to overcome IDC is greatly reduced. The greater the salinity, the worse will be the IDC if carbonate content predisposes the plant to IDC.

Therefore the number one management strategy in the region to reduce IDC is field selection. Fields that are 7 and below in pH are preferred. The lower the salt (EC) values, the better. Once a field hits EC 1.5, bad things will happen to soybeans. An EC of 2? Call your insurance agent the day after you plant.

The second most important strategy is variety selection. Dr. Kandel and Dr. Helms maintain a site with regional IDC ratings. Using local ratings is very important because these varieties are screened on sites with both carbonates and soluble salts, whereas Iowa ratings are screened solely on carbonate tolerance, with little or no soluble salts.

Other management aids are 1.) planting in wider rows to decrease distance between plants, 2.) seeding a cover crop of oats, barley or rye at the time or before time of seeding to help dry the soil and take up some soil nitrate (high soil nitrate, which we had this spring, causes a physiological precipitation of iron inside the leaf, rendering within-plant iron unusable), and 3.) use a high ortho-ortho-EDDHA containing iron fertilizer with the seed at planting.

Spraying a foliar iron fertilizer at this time will only affect leaves that are out and new leaves will come out yellow. Foliar iron is not mobile within the plant.

When the soil dries out, most areas of IDC will become green again. Some areas that remain moist due to a high water table, and have high soil carbonate and salts, may not green up at all depending on variety or even in spite of the variety.

For next year, have fields screened for CCE (calcium carbonate equivalent) and EC (soluble salts). Choose the most favorable fields to seed soybeans for greatest economic return in 2019.

Dave Franzen

Extension Soil Specialist

701-799-2565

This site is supported in part by the Crop Protection and Pest Management Program [grant no. 2017-70006-27144/accession 1013592] from the USDA National Institute of Food and Agriculture. Any opinions, findings, conclusions, or recommendations expressed are those of the website author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture.

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