A relatively early harvest throughout most of Illinois should allow enough time to get wheat planted and well established before it gets cold, says Fabian Fernandez, University of Illinois Extension specialist in soil fertility and plant nutrition.
To ensure adequate nutrient availability and successful establishment, he offers a few suggestions for growers.
Nitrogen is important for vegetative growth, but the amount taken up by roots and vegetative tissues does not exceed 30 to 40 pounds of nitrogen per acre before it gets too cold. While concern about lodging under high nitrogen rates has decreased considerably, it is best not to apply too much nitrogen in the fall to minimize lodging risks.
Also, because nitrogen applications will promote excessive vegetative growth, the crop may be prone to disease problems later, he adds. If the soil has large potential to supply nitrogen, fall applications prior to planting may not be necessary.
"Corn fields should have some leftover nitrogen in the soil this year as the corn crop probably did not use all the nitrogen during dry conditions in July and August," Fernandez adds. "Normally, a 20-to-30-pound-nitrogen-per-acre application in the fall is all that is needed to establish wheat. This amount can be supplied in the form of di-ammonium phosphate, which should also supply what is needed for phosphorus fertility."
The total amount of nitrogen required for a wheat crop is dependent on the capacity of the soil to supply nitrogen. Dark soils high in organic matter require less nitrogen than light-colored soils with low organic matter.
For soils with organic matter greater than 4%, 70 to 90 pounds of nitrogen per acre is typically sufficient; soils with organic matter between 2% and 4% often maximize yields with a rate of 100 to 120 pounds of nitrogen per acre; and soils with low or less than 2% organic matter will require 150 pounds of nitrogen per acre.
"While the full amount can be applied with anhydrous ammonia and a nitrification inhibitor in the fall, the preferred method is to apply most of the needed nitrogen by topdressing with fertilizers that do not contain free ammonia in the spring right before the crop greens up and starts to take nitrogen," he says. "Applying nitrogen at this later time minimizes the potential for nitrogen loss, and provides needed nitrogen that might not be available from the soil due to slow mineralization during cool springs."
In high organic matter soils, spring application timing is not as critical. In low organic matter soils in southern Illinois it is possible to reduce nitrogen rates by 10% by delaying application to late tillering (Feekes growth stage 5 to 6), splitting the application between early green-up and late tillering to early jointing, or using a nitrification inhibitor or slow-release nitrogen source.
While most wheat is planted after soybeans, if wheat is planted after corn, one potential concern is the temporary tie-up of nitrogen that can occur as 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 nitrogen. For these reasons, additional nitrogen beyond the recommendation is not needed for wheat grown after corn.
Phosphorus is very important to stimulate early growth, help with tillering, and improve winter survival. The amount of phosphorus to be applied depends on the soil test levels as well as the phosphorus-supplying power of the soil. It is recommended that the soil test level be at 40, 45, and 50 pounds per acre for the high-, medium-, and low-phosphorus-supplying power soils, respectively.
If the soil test is below the desired level, it is recommended to apply sufficient phosphorus to build up the soil as well as supplying what the crop will remove. If test levels are adequate, it is recommended that sufficient phosphorus be applied 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 phosphorus requirements of wheat.
"In many fields, a typical rate of 150 pounds of DAP (18-46-0) per acre supplies not only phosphorus, but also sufficient nitrogen for the establishment of the crop," Fernandez says. "Sometimes it might be tempting to reduce or eliminate phosphorus application in soils testing at or just above the critical level. If your finances do not allow for a full application, it is strongly suggested that 80 to 100 pounds of DAP per acre be applied to ensure a good supply of readily available phosphorus to help adequate establishment of the crop."
Potassium is also an important nutrient, but wheat normally does not respond to applications of this nutrient unless soil test levels are extremely low (less than 100 pounds per acre). Since soybean and corn are grown in the rotation with wheat, and are more responsive to potassium than wheat, it is recommended to manage this nutrient to maximize yield of corn and soybean, he adds. This will automatically take care of the potassium needs of wheat.
For more information, read The Bulletin online at http://bulletin.ipm.illinois.edu.