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Managing iron deficiency chlorosis in soybean

Soybean plants showing signs of iron deficiency chlorosis

Iron deficiency chlorosis in soybean, otherwise known as IDC or iron chlorosis, is a problem for soybean production in South Central, Southwest, West Central and Northwest Minnesota. The primary symptom is interveinal chlorosis (yellowing) of the leaves with the leaf veins remaining dark green.

Iron (Fe) is required by several enzymes involved in the formation of chlorophyll and lack of active iron in leaves leads to chlorosis. Soil normally contains large amounts of iron, but it is not in the soluble form needed by the plant. The most soluble form in oxidized (aerated) soils is Fe(OH)3, where iron is in the Fe(III) form. This iron becomes less soluble at higher soil pH, especially when the soil has large amounts of calcium carbonate. 

Plants prefer to take up the reduced form of iron (Fe II) and have adapted mechanisms to help extract iron from the soil.

  • Type I plants, such as soybean, azaleas and blueberries excrete acids and chemical reductants from their roots. The acids make the Fe(OH)3 more soluble and the reductants change insoluble Fe(III) to more soluble Fe(II).
  • Type II plants such as corn and grasses excrete iron chelators that bind Fe(III)  allowing the iron to be absorbed through the root.

Plants vary in their ability to extract Fe from the soil. Azaleas and blueberries only survive in acidic soils where Fe(OH)3 is more soluble. Because of this, azaleas and blueberries are chlorotic in soils with a pH greater than 5.5 while soybeans can adequately grow when pH is less than 7.5.

Aggravating conditions for IDC

Type I plants and high lime (calcium carbonate) IDC

High lime IDC is quite common for soybean, peanut, grapes, citrus and peaches. The use of a soil test for iron will generally not indicate these problems.

High lime IDC is more severe in soils with finely divided lime (calcium carbonate). The fine calcium carbonate particles contact the soybean root, slowly neutralize the excreted acid that is meant to solubilize iron in the soil and inhibit the plant from taking up iron that is in the soil. 

Wet soil

When soil is wet, there is limited air exchange with the atmosphere causing a buildup of carbon dioxide in the soil that is produced by roots and soil microbes through respiration. The amount of bicarbonate in the soil is proportional to the amount of carbon dioxide, and as carbon dioxide increases so does bicarbonate. This increase will rapidly neutralize the acidity around the soybean root.

Research in the greenhouse has shown that the severity of IDC increases for soybean growing on waterlogged soils, and especially when soils are cold. The amount of bicarbonate in the soil has been correlated with IDC in soybean in the field, so the more bicarbonate in the soil the more IDC will occur.

In wet soil conditions, it may be useful to consider cultivation to speed the release of carbon dioxide from the soil.

Addition of fresh organic matter

Decaying organic matter, whether it is from crop residues or manures, adds to the amount of carbon dioxide in the soil as microbes release carbon dioxide. If soil is wet, diffusion of the carbon dioxide out of the soil will be limited, increasing the risk for buildup of bicarbonate in the soil and therefore increasing the severity of IDC.

Nitrate in the soil

Green wheel tracks through an IDC-affected area of the field

Soil nitrate in the field can increase chlorosis. Field and greenhouse studies have shown the addition of nitrate increases the severity of chlorosis in soybean.

Nitrate can affect IDC in two ways.

  • First, when a plant root takes up nitrate it must exchange with a bicarbonate ion.
  • Second, nitrate taken up into the plant must be converted to ammonium within the leaves, which increases pH in leaf sap and decreases the rate of reduction of Fe III to Fe II, which is necessary for leaf cells to have usable Fe.

When taking plant samples from chlorosis fields, iron in leaflets can be greater in the chlorosis-affected plants than non-chlorotic plants because of an accumulation of iron in the leaves without the ability to reduce the iron III into an available form. 

Differences in soil nitrate may explain why we often see less chlorosis in the wheel tracks of the secondary tillage in a soybean field.

  • The soil under the tractor wheels is more compacted, causing the compacted area to be more anaerobic (low air), but not low enough to reduce much Fe III to Fe II.
  • The slightly anaerobic soil conditions can cause denitrification of nitrate to N2, reducing the amount of nitrate in the soil and thus reducing the amount of IDC.

The effect of soil nitrate-N on IDC and soybean grain yield

N rate (lb/acre) Oat companion crop C06 grain yield (bu/acre) YM06 grain yield (bu/acre) K07 grain yield (bu/acre) YM07 grain yield (bu/acre)
0 No 42.1 52 3.6 51.7
100 No 28.5 32.2 0.3 46.5
200 No 25.3 19.1 0.1 40.2
0 Yes 42.5 52.4 52.4 50.7
100 Yes 20.5 42.6 42.6 43.4
200 Yes 18.9 25.9 25.9 33.7

Plant stress

Any production practice that can cause stress to the plant can also cause problems with iron uptake by the plant. Examples include herbicide injury, (especially from soil-applied products), plant disease pressure and nematodes.

Soybean cyst nematode (SCN) populations can be very high in field areas where IDC occurs. While both maladies can cause stunting and yellowing of soybean, research in Minnesota has not shown an interaction between SCN and IDC. Management strategies for both SCN and IDC should be considered when these exist within the same field.

Where we find IDC in Minnesota fields

Typical soil association from the western Minnesota prairie pothole area. IDC typically is found in low-lying areas (Okoboji and Harps soils), where salts and carbonates accumulate over time.

Soybean IDC generally occurs in shallow depressions. IDC is generally worse on the rims of the potholes where higher concentrations of calcium carbonate have been deposited, caused by historic soil conditions when the soils were wet prairies before tile drainage and farming.

The intensity and extent of IDC depends on the soybean variety and factors that affect soil nitrate and bicarbonate contents.

How to manage IDC


Tailor practices to your farm

Research involving the combination of tolerant variety, companion crop, seed placed ortho-ortho chelate, and soybean seeding rate has demonstrated that each can be used to reduce the impact of IDC. But IDC is a process that is impacted by both soil and environmental factors.

While all factors together may potentially benefit soybean by reducing IDC severity, you may not need to do all of these things to reduce IDC severity.

From both a yield and economic perspective, the management practices that were most beneficial for producers were variety selection, followed by a seed placed ortho-ortho chelate, increased seeding rate, and lastly the companion crop.

Farmers should tailor these practices to their farms or parts of farms based on risk of loss from IDC as well as their own logistical, mechanical or management limitations.

Authors: Daniel E. Kaiser, Extension nutrient management specialist, and Seth L. Naeve, Extension soybean agronomist

Reviewed in 2023

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