Iron deficiency chlorosis (IDC) is a serious problem when soybeans are grown on high-pH soils.

“Chlorosis in soybeans can occur whenever soybeans are grown on soils with calcium carbonate, or lime, in the topsoil,” says R. Jay Goos, a professor in the Soil Science Department at North Dakota State University. “Lime in the topsoil can occur on eroded soils, where calcareous subsoil is exposed.”

However, in North Dakota, IDC is more common on level, poorly-drained soils.

“Soils with a high water table in the spring are the soils most associated with IDC,” he says. “These soils often have elevated salinity as well, which makes chlorosis worse. Most of the crops we grow aren’t bothered by the presence of lime in the topsoil, but soybeans can be. The soybeans will develop a yellow color, and if the deficiency progresses, the growing point will be damaged.”

The most practical control measure is choosing a resistant variety.

“No soybean variety is immune to chlorosis, but there are huge differences between the most resistant and most susceptible varieties,” Goos warns. “The wrong choice can lead to reduced yield, or even crop failure.”

Since 2001, Goos, with the financial assistance of the North Dakota Soybean Council, has screened soybean varieties for resistance to chlorosis.

“We test the soybeans on four or five field sites per year, with multiple replications and measurements,” he says. “When we put out a chlorosis score, there are typically 40 observations behind each score. In general, if a variety is planted in an NDSU yield trial, it gets a chlorosis evaluation.”

More than 350 varieties were tested in 2008.

The method of reporting the results has changed for 2008.

“In the past, we have simply reported the average of our chlorosis scores,” Goos says. “We are still doing this, but we are also experimenting with putting out the scores in terms of a ‘report card,’ which ranks the varieties relative to the other varieties in the marketplace.”

The problem, according to Goos, is that chlorosis scores, like yields, vary from year to year. A score of 2.5 might be a relatively good score in one year, but only a mediocre score in another year. By giving a variety a grade, at least a producer can know how well a variety did, compared with the other varieties in the marketplace.

Goos suggests the following guidelines for interpreting the chlorosis report card:

• Avoid the D’s. These varieties are too susceptible to plant on chlorosis-prone soil.
• You can do better than a C. Stronger varieties are available for chlorosis-prone soil.
• A grade of B or B- is a good grade. However, for fields with the worst chlorosis problems, stay with varieties with a score of A, A- or B+. By sticking with these varieties, producers are going with the top 10 percent of the marketplace for resistance, which they will need for the toughest conditions.

Goos cautions that these trials do not apply to fields without chlorosis problems.

“Our ratings are only for high-pH fields with chlorosis problems,” he says. “For well-drained fields without lime in the topsoil and no history of IDC in soybeans, the chlorosis resistance score of a variety is of little concern.”

The 2008 trial results are posted on Goos’ Web site at http://www.yellowsoybeans.com.

NDSU Agriculture Communication