No. 22 Article 6/September 23, 2011

Nutrient Applications for Wheat

Relatively early harvest in most of Illinois this year should allow enough time to get wheat planted and well established before it gets cold. If a wheat crop is in your plans this fall, consider a few pointers to ensure adequate nutrient availability and successful establishment of the crop.

Nitrogen (N) is important for vegetative growth, but the amount taken up by roots and vegetative tissues does not exceed 30 to 40 lb N per acre before it gets too cold. Nearly all modern varieties of wheat have been selected for improved standability. While concern about lodging under high N rates has decreased considerably, to minimize any lodging risks it is best not to apply too much nitrogen in the fall. Also, because N applications will promote excessive vegetative growth, the crop may come across disease problems later.

If the soil has large potential to supply N, fall applications before planting may not be necessary. Also, this year's cornfields should have some leftover N in the soil, as the crop probably did not use it all during dry conditions in July and August. Normally, in any given year an application of 20 to 30 lb N per acre in the fall is all that is needed to get wheat established. This amount can be supplied in the form of di-ammonium phosphate (DAP), which should also supply what is needed for phosphorus fertility (see the later discussion).

The total N required for a wheat crop depends on the capacity of the soil to supply it. Dark soils high in organic matter require less N than light-colored soils with low organic matter. For soils with organic matter higher than 4%, 70 to 90 lb N per acre is typically sufficient; soils with organic matter between 2% and 4% often maximize yields with a rate of 100 to 120 lb N per acre; soils with organic matter of less than 2% require 150 lb N per acre.

While the full amount of N can be applied with anhydrous ammonia and a nitrification inhibitor in the fall, the preferred method is to apply most of what is needed by top-dressing in the spring, right before the crop greens up and starts to take N. Application at this later time minimizes the potential for loss and provides needed N that might not be available from the soil due to slow mineralization of soil N by bacteria during cool springs. The top-dressing can be accomplished with dry or liquid N solutions as long as they don't contain free ammonia. If you use urea, it is important to apply it when leaves are free of dew or moisture and the soil surface is not excessively dry.

While most wheat is planted after soybean, if it is planted after corn, one potential concern is that N can be temporarily tied up while microorganisms break down corn stover. Fortunately, most of this tie-up takes place in the spring once soils warm up, which is often after wheat has taken up most of its N. For these reasons, N beyond the recommended amount is not needed for wheat grown after corn.

On soils with higher organic matter, spring application timing has little impact on yield. On the other hand, N rates can be decreased by 10% in soils low in organic matter in southern Illinois when one of the following applies:

Research has also shown that a spring-split N application, with one-third early and two-thirds at late tillering to jointing, can increase yields by about 10% compared to a single spring application at greenup, especially when conditions favor N loss. Delaying all of the application to late tillering or early jointing usually produces the same yield as splitting applications in the spring.

Phosphorus (P) is very important to stimulating early growth, helping with tillering (which eventually determines the number of seedheads), and improving winter survival. The amount of P to be applied depends on soil test levels as well as the P-supplying power of the soil. It is recommended that test levels for high, medium, and low P-supplying soils be at 40, 45, and 50 lb per acre, respectively.

If P is below the desired level, it is recommended that you apply enough to both build up the soil and supply what the crop will remove. If test levels are adequate, it is recommended that you apply enough at planting time to replace 1.5 times the amount to be removed by the crop. This large amount is needed to meet the high P requirements of wheat. In many fields, a typical rate of 150 lb of DAP (18-46-0) per acre supplies not only P but also sufficient N for establishment of the crop (discussed earlier). It might be tempting to reduce or eliminate P application in soils that test at or just above the critical level. If your finances do not allow for a full application, it is strongly suggested that you apply 80 to 100 lb of DAP per acre to ensure a good supply of readily available P to facilitate adequate establishment of the crop.

Potassium (K) is also an important nutrient, but wheat normally does not respond to applications of K unless soil test levels are extremely low (<100 lb per acre). Since soybean and corn are grown in the rotation with wheat and are more responsive to K than is wheat, it is recommended that you manage K to maximize yield of corn and soybean. Doing so will automatically take care of the needs of wheat.--Fabián G. Fernández

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