The Curious Case of Yellow Soybeans

A farmer in Coles County, Ill., told AgWeb on June 29 how their soybean crop has slowly turned yellow. Will it recover? How prevalent is this problem?

When you’re driving down the road, a yellow light signals caution, and the same thing goes for your soybean crop. If you don’t give them the proper nutrients, they will turn yellow and possibly die.

“In Iowa, you will often see iron chlorosis and potassium deficiency,” says John Sawyer, a soil fertility specialist at Iowa State University.

Iron Chlorosis

Iron chlorosis is characterized by the youngest leaves of the plant turning yellow while the veins remain green, and is often seen earlier in the season, Sawyer says. It is commonly found in soils with a high pH as well as very wet soils with high nitrate levels.

Different varieties of soybeans are available, and Sawyer recommends purchasing one with a higher tolerance to iron chlorosis.

“The symptoms will show in the same field areas as they have in the past,” Sawyer says. “You can target these areas and plan to plant a more tolerant variety.”

Other research shows areas of more compact soil have helped alleviate iron chlorosis, says Ignacio Ciampitti, a crop production specialist at Kansas State University. A higher plant population can also help reduce high nitrate levels and the deficiency symptoms, he notes.

Potassium Deficiency

Potassium deficiency is often seen in older leaves of the plant later in the season. It starts to yellow at the tips and slowly work along the edges. Deficiencies are often observed in soil areas where potassium is not readily available to the plant or there are insufficient supply levels.

“Soil testing is the best recommendation avenue for the best treatment of the crop,” Sawyer says.

This means there isn’t a truly effective treatment for a potassium deficiency, so the best way is to be proactive about it. This should include testing the soil before planting and preparing a potassium fertilization program.

Although compact soil may help iron chlorosis, Ciampitti says it can limit root growth and further harm potassium levels of the plants.

Nitrogen Deficiency

One other common nutrient deficiency in soybeans is nitrogen. With this condition, the older leaves of the plant turn yellow because any available nitrogen has been diverted to younger leaves.

In wet conditions, rhizobial node formation is delayed and can’t get the plant enough nitrogen, says Dorivar Ruiz Diaz, a Kansas State University nutrient management specialist. Once the soil dries out the rhizobia will be able to get back to work and the symptoms will stop.

If the symptoms persist, Diaz recommends treating the plants with a small amount of nitrogen. It also helps to be proactive and treat the seed before planting with an inoculant.

“Historical recommendations have always been [use an inoculant] if a field has been out of soybeans for three or more years,” says Jeremy Ross, a University of Arkansas soybean specialist.

Using an inoculant before planting gives the soybean a dose of rhizobia and ensures successful nitrogen fixation and growth.

Nutrient deficiencies can be hard to treat at times. If this is the case, Sawyer says the best thing to do is wait until the soil dries out and hope plants recover without intervention.