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  AGRONOMIC NEWS ITEMS
From Agronomists of the
Potash & Phosphate Institute
655 Engineering Drive, Suite 110
Norcross, Georgia 30092-2837
Phone (770) 447-0335

Winter 1999, No. 7

TOPDRESSING WINTER WHEAT: NITROGEN AND BEYOND

    A key to successful fertility management in crop production involves having adequate fertility available when the plant needs it. Splitting applications of some nutrients between preplant and late winter or early spring is a practice that in many cases can add yield and profit to winter wheat production. When topdressing winter wheat is mentioned the first thing to come to mind is usually nitrogen fertilization. However, the need for other nutrients, especially those that are susceptible to leaching from soils, should be considered along with nitrogen. These nutrients include chloride, sulfur, and in some cases potassium. With that, lets look at some considerations in winter wheat topdress fertilization.

    Wheat takes up about 2.0 to 2.5 pounds nitrogen per bushel produced, or if grazed, about one pound for 3 pounds of animal gain. Adequate nitrogen must be available to the wheat plant at all phases of development. Shortages may ultimately result in reduced tillering, reduction in head size, poor grain fill, reduced yields, and low protein content. Splitting nitrogen applications generally improves use efficiency, minimizes risk to investment, and safeguards the environment. This is especially true in medium to light textured soils and in higher rainfall regions where the risk of leaching is increased. Topdress applications should be made early, prior to jointing, to maximize production efficiency. Timing and source should be managed to fit climatic conditions, soil type, and tillage system.

    Another nutrient that should be considered in topdressing winter wheat is chloride. There have been numerous studies across North America in the past several years that have evaluated wheat response to chloride. Chloride is essential for photosynthesis, it is important in controlling the opening and closing of leaf stomata (pores), it influences the nitrogen nutrition of plants, and also advances plant maturity and improves overall disease resistance. Wheat response to chloride is usually expressed in improved color, suppression of fungal diseases, and increased yield. The average yield increase due to chloride is usually about 5 bushels per acre in responsive conditions, although yield increases as high as 23 bushels per acre have been observed. Chloride is highly mobile in soils so split or topdress application may be beneficial in regions of sufficient precipitation to cause leaching. Several factors and tools can be used to determine whether a specific wheat production situation is likely to be responsive.

    In well-drained sandy soils, and low organic matter soils sulfur may limit wheat yield and profitability.  Sulfate sulfur, like chloride, is an anion and therefore mobile in soils and subject to leaching. Because of its mobility, topdress applications may be effective in addressing sulfur needs. In sandy well-drained soils potassium may also be limiting. If sufficient preplant potassium was not applied, or if excessive leaching has occurred, topdress application can be effective in correcting deficiencies.

    Providing adequate fertility when it’s needed is an important objective in any wheat production scheme. Surface or topdress application in the spring with some nutrients is agronomically sound because of the mobility of these nutrients in soils. So, look beyond nitrogen this topdress season and consider the need for nutrients such as chloride in wheat production.


—WMS—

For more information, contact Dr. W.M. (Mike) Stewart, Great Plains Director, PPI, P.O. Box 6827, Lubbock, TX 79493. Phone: (806) 795-3252. E-mail: mstewart@ppi-far.org.
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