For the past five years, R. Jay Goos, North Dakota State University soil science professor, has been screening soybean varieties for resistance to chlorosis under field conditions.

The project evaluates about 180 varieties per year, including commercial and university varieties. The results of the 2005 trials and the results for prior years are available online at www.yellowsoybeans.com

Iron deficiency chlorosis is a common problem when soybeans are grown on alkaline soils.

"Chlorosis is usually a problem associated with poorly-drained, high pH soils," Goos says. "Soybeans are particularly sensitive to such conditions. Many fields that grow heavy crops of wheat or barley can give rise to soybeans that are yellow and stunted. Even if the soybeans recover, the yield is reduced. Even a temporary yellowing can cut yields 20 percent."

Goos has evaluated many control measures for chlorosis during the past eight years.

"By far, the most practical control measure is the selection of a soybean variety that is more resistant to chlorosis and better at acquiring iron from the soil," Goos says. "Foliar sprays with iron often are not capable of correcting the disorder once it sets in, and effective soil-applied fertilizers are too expensive. Using a heavy seeding rate with soybeans planted in 30-inch rows reduces chlorosis, but the seeding rates required are too expensive for farmers using transgenic seed. We simply haven't been able to identify a control measure that is as effective and economic than the selection of a resistant variety."

The trials were sponsored by the North Dakota Soybean Council.