Northwest Oklahoma wheat is in various stages of growth. We have five- and six-tillered wheat stocker cattle have been placed on, to small wheat that has only three or four leaves.

We also have, thanks to Mother Nature, wheat that is still in the bag or on the truck waiting to be sown. Given this amount of diversity, my comments here fit some operators while others may not experience what I have seen while visiting fields in the northwest portions of the state.

As I travel, I am seeing emerged wheat with a light green appearance. I am seeing the older leaves of the wheat plant begin to turn yellow at the tip and then move to midrib of the leaf in a V-shaped pattern. Eventually, the entire lower leaf of the wheat plant will turn yellow and then brown. At this time we also are seeing the same pattern develop on the second and third oldest leaves. Those leaves turn yellow and then brown and die. Meanwhile, at the top of the plant we are seeing green color in the newest leaves. This is classic nitrogen deficiency.

As I review these characteristics, a producer might say he had put down 100 pounds of actual nitrogen in the form of anhydrous ammonia. There is no way the plant could have used all that nitrogen by now. The producer is in fact correct. The plant might not have been able to utilize the nitrogen because it has moved lower in the soil profile and is unavailable to the plant roots.

Nitrogen can be lost from the wheat production calendar in four major ways. It can volatilize as ammonia. Nitrates can, in poorly aerated soil, be lost as nitrous oxide or nitric oxide (denitrification). Nitrogen can be leached through the soil with water movement. Or nitrogen can be lost through the plant stomata in some of its respiration activities. Of the before mentioned loss options, the two we are most concerned with at this time would be denitrification and leaching.

In water logged soils, soil microbes need oxygen to live. Without oxygen in the soil profile, these microbes will strip away oxygen from the nitrate molecule leaving the soil with either nitrous oxide or nitric oxide which is released from the soil in a gaseous form and into the atmosphere. This nitrogen is gone and cannot be recaptured by the growing wheat plant.

Wheat plants take nitrogen from the soil either as the ammonium ion or as nitrate. The ammonium ion exists for only a fraction in time so the nitrate form is the most common form of entry into the plant. In the soil profile nitrates are mobile. This means the nitrates will move with water. As the soils dry out and the plant is pulling water from the soil, nitrates move to the root to be utilized in the growth processes. On the other hand, if water is moving downward though the soil profile, the water takes nitrates away from the root zone where the plants need nitrates. In time, as the soils dry out, some of the lost nitrates will return to the root zone and be utilized by the plant.

In short, producers are seeing yellowed wheat. The reason for the yellowing is a loss of nitrogen in the soil profile due largely to the amount of rainfall received over the last 45 days or so. As we dry out, most of the nitrogen will be regained, but there are some locations where we have lost nitrogen and at some point in time, the producers will have to add additional nitrogen to the production system to maintain enough nitrogen for the yield goal of the producer.

If you have questions or concerns with this yellowing pattern, please feel free to contact the Oklahoma Cooperative Extension ag educator in your county. They can assist in evaluating your crop and your nitrogen needs for the year.



Gribble is Oklahoma Cooperative Extension Service northwest area agronomist.