Issue No. 23, Article 7/October 3, 2008
Fertilizing the Wheat Crop
Wheat seeding is going to be late in many acres this year because of delayed harvest of the summer crops. Many wheat acres are planted following soybean because there are fewer problems with disease and stands are established more successfully than when planted following corn. The soybean crop is especially late this year, mostly due to late planting extending into June. If weather conditions are close to typical this fall, wheat will have to get established in a short period before temperatures are too cold for growth. In addition to selecting the best hybrid and possibly increasing plant population to compensate for late planting, this year it will be critical to help the crop become well established and to develop a good number of tillers in the fall by ensuring an adequate supply of nutrients.
Nitrogen (N) is important for vegetative growth, but the amount needed in the fall is minimal (typically less than 30 lb per acre). The reason for this is that the crop normally has a short time for growth before it becomes too cold. In addition, it is not desirable to promote excessive vegetative growth in the fall because of potential disease problems in the spring as well as potential lodging (although this is not as big of a problem as it used to be with older hybrids). If the soil has large potential to supply N, fall applications prior to planting are not necessary. The total amount of N required for a wheat crop depends on the capacity of the soil to supply N. Dark soils high in organic matter require less N than light-colored soils with low organic matter. For soils with organic matter greater than 3%, 50 to 70 lb per acre is typically sufficient; soils with organic matter between 2% and 3% often maximize yields with a rate of 70 to 90 lb per acre; and soils with low organic matter (less than 2%) will require 90 to 110 lb 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 N by top-dressing in the spring right before the crop greens up and starts to take N. Applying N at this time minimizes the potential for N 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 do not contain free ammonia. If urea is used, it is important to apply it when leaves are free of dew or moisture and the soil surface is not excessively dry.
Phosphorus (P) is very important to stimulate early growth, help with tillering (which eventually determines the number of seed heads), and improve winter survival. The amount of P to be applied depends on the soil test levels as well as the P-supplying power of the soil. It is recommended that the soil test level be at 40, 45, and 50 lb per acre for soils supplying P at high, medium, and low levels, respectively. If the soil test is below the desired level, it is recommended to apply sufficient P to build up the soil as well as to supply what the crop will remove. If test levels are adequate, it is recommended that sufficient P 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 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. With high fertilizer prices, it is tempting to cut or eliminate this 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 consider at least applying 80 to 100 lb of DAP per acre to ensure a good supply of readily available P to assist with adequate establishment.
Potassium (K) is also an important nutrient, but wheat normally does not respond to applications of K unless soil test levels are extremely low (less than 100 lb per acre). Since soybean and corn are grown in the rotation with wheat and are more responsive to K than wheat, it is recommended that you manage this nutrient to maximize yield of corn and soybean. At this time of high fertilizer prices, reducing or eliminating K applications for wheat is most likely the action with the least potential to negatively impact the wheat crop.--Fabián G. Fernández