Tree leaves aren't the only thing turning yellow this fall. There are also plenty of wheat fields with a yellowish cast, especially in central Kansas, said Dave Mengel, Kansas State University Research and Extension soil fertility specialist.

The most common cause of yellow wheat is nitrogen deficiency, Mengel said. One of the reasons for nitrogen deficiency on wheat in the fall is nutrient tie-up, or nitrogen “immobilization,” on plant residue, he said.

Nitrogen immobilization is common where less than 25 pounds per acre of fertilizer nitrogen has been applied to the wheat, and there are large amounts of wheat, corn, or sorghum residue present in the upper layer of soil or on the soil surface.

“The amount of undecomposed plant residue present is a key factor. Immobilization of nitrogen is more likely when there are high levels of plant residue with a wide carbon-to-nitrogen ratio. Wheat, corn and sorghum residue all have a wide carbon-to-nitrogen ratio. These plant residues are an energy source for the microorganisms commonly found in soils,” Mengel said.

“When plant residues are present, soil microbes actively begin to use them as a food source,” he added. “The soil microbes begin multiplying, and in the process they utilize nitrogen present in or on the soil. The microbes incorporate nitrogen into proteins, nucleic acids, and other organic nitrogen compounds.”

This nitrogen essentially becomes part of the soil organic biomass, and will remain unavailable to plants until the carbon-to-nitrogen ratio drops to a critical ratio, about 20-to-1, the soil scientist explained.

“At that point, nitrogen is no longer limiting and some of it will become available to plants again through the process of mineralization,” Mengel said.

Until the crop residues have been sufficiently decomposed, nitrogen will remain tied up in the microbes. During this period, wheat plants may show nitrogen deficiency even if fertilizer nitrogen was applied, especially if the amount of fertilizer nitrogen applied was too low.