NEWS for North Dakotans
Agriculture Communication, North Dakota State University
7 Morrill Hall, Fargo, ND 58105-5665

December 10, 1998

Iron Chlorosis Problem for Soybean Growers

Soybeans are a new crop for many growers. Some of these growers get an unpleasant surprise when soybean plants turn yellow in fields that have produced good yields of other crops in the past.

According to Jay Goos, soil scientist at North Dakota State University, the problem is most often iron chlorosis, a yellowing of leaves because plants can't make chlorophyll due to a shortage of iron.

Iron is actually abundant in the soil, and only a small amount is needed by plants. Unfortunately, iron is very insoluble in the soil, and solubility declines as soil pH goes up, becoming very low as the pH goes above 7. High pH is typical of North Dakota soils. Wet soils, as has been typical in many areas of North Dakota in recent years, also makes the problem worse.

Soybeans are particularly inefficient in using iron compared to other crops, Goos says. "Growers who have successfully grown other crops for decades are often surprised when soybeans turn yellow on these same fields. Species vary greatly, and there are also varietal variations."

Crops like barley and wheat are efficient in taking up iron, and seldom show iron chlorosis. Crops like sunflower, corn, and dry beans are intermediate, and show chlorosis under some conditions. However, soybeans are particularly sensitive to this problem. Fields that have produced excellent yields of wheat or barley in the past may be a problem with soybeans.

Goos says field selection is the first consideration for a soybean grower. Not all fields are appropriate for soybean production. Soils with poor drainage, high pH, and salinity may pose problems for soybeans that can't be overcome with resistant varieties or application of supplemental iron.

If soybeans are going to be grown on a high pH field, variety selection is the most important management decision a farmer can make. Some varieties are more resistant to chlorosis problems, although no variety is immune. The more resistant varieties don't turn as yellow, and don't get as "sick." With a resistant variety, the youngest leaves and growing point are not as likely to die, giving more opportunity for recovery or response to foliar application of iron.

Varieties more resistant to chlorosis problems include P9004, P9071, Council, Dawson, McCall and Traill. Varieties that are susceptible to iron chlorosis include Maple Amber, Maple Glen, Glacier and P9092.

No soybean variety approaches the resistance to chlorosis of other crops such as barley, Goos says. He advises growers to ask seed dealers, Extension Service staff or crop consultants about varieties that are most resistant to iron chlorosis.

Foliar application of iron, especially in conjunction with chlorosis-resistant varieties, can be an effective treatment, says Goos. Leaf absorption of iron is good, and re-greening of plants is often rapid if the plants are not already too sick. The treatment must be done early to be effective.

"Early application means as soon as yellowing begins...even at the first trifoliate stage. If spraying is delayed, the plant's growing point may be damaged, and spraying will do little good," he says.

Spray treatments are relatively inexpensive and deserve more attention from growers, he says. For example, a foliar application could easily be done in conjunction with cultivation, where soybeans are grown as a row crop.

The source of iron used for foliar spraying is not critical. Iron lignosulfonate, iron glucoheptonate, iron EDTA, and iron HEDTA are liquid iron products commonly sold by dealers for foliar spraying, and all are effective.

Seed treatment with iron fertilizers has some potential for the future, again being most effective with resistant varieties, Goos says. It gives an early-season boost and may keep plants from becoming so sick, allowing them to recover later in the summer. At this time, only one iron fertilizer, FeEDDHA, is known to be effective for seed treatment. One published study in Minnesota showed that coating the seed of chlorosis tolerant varieties at the rate of a pound of FeEDDHA per 100 pounds of seed increased the yield by about 6 bushels per acre under conditions of iron chlorosis.

Methods of applying iron compounds on seed need to be developed, however, and Goos does not know of any seed dealers who currently offer this treatment.

Applying FeEDDHA with the seed of soybeans can also reduce early-season chlorosis. Dissolving 0.5 to 1 pound per acre of FeEDDHA in water and placing with the seed can be helpful, particularly with 30-inch rows.

Unfortunately, the cost of FeEDDHA prevents the use of higher rates, Goos says.

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Source: Jay Goos (701) 231-8581

Editor: Gary Moran (701) 231-7865