Nitrogen is one of the major nutrients required by forage grasses for proper growth and development. Yield and forage quality response to added nitrogen can be dramatic. But unlike the other two major nutrients, phosphorus and potassium, nitrogen is not retained in the soil from year to year in a form that forage plants can readily use.
In forage systems without legumes or clovers, and in some legume grass management situations, nitrogen must be added as a fertilizer material to achieve the best forage grass production and quality. Several types of nitrogen fertilizer materials are available. The best one for you will depend on factors such as availability, price, equipment and time of year.
When nitrogen is applied to the soil it is rapidly converted to nitrate-N. Nitrate-N is rapidly available to the forage plant but, especially in most Wilson County soils, nitrate-N can move with the soil water out of the crop-rooting zone. Also in wet or poorly drained soils, the nitrate-N can be converted back to gaseous forms and lost to the atmosphere.
Urea is perhaps the most commonly available and used nitrogen source in Tennessee. It contains 45 percent actual nitrogen or 45 pounds of nitrogen per 100 pounds of fertilizer material. Urea has good handling and storage properties because it does not absorb water as quickly as ammonium nitrate. These properties enable fertilizer dealers to store and process this material during humid weather, making it a material of choice for bulk storage and blending operations. Urea is generally one of the lower-priced nitrogen fertilizer materials.
To minimize potential for nitrogen loss, all inorganic nitrogen fertilizer materials should be applied close to the time of expected forage production. Volatilization loss is of particular consideration for urea containing fertilizer materials. Research shows that the potential for nitrogen loss as ammonia gas increases as temperature, soil pH and moisture increase, and as rate of application increases. The percent of surface-applied nitrogen from urea volatilized can increase about four-fold as the soil temperature increases from 45 to 90 degrees. This information suggests that fertilization of cool-season forages in March would result in lower potential for nitrogen loss than might be realized for fertilization of warm-season forages or for fertilization of fescue in August.
Volatilization loss can be significantly reduced by addition of a urease inhibitor to the fertilizer material. This increases the price of the material, but the cost is usually still lower than or equal to that of ammonium nitrate. Potential for foliage burn with urea materials is low, and when incorporated into the soil by rainfall or tillage, it is as efficient a source of nitrogen as any source, even without the use of a urease inhibitor. Although burn potential is low, application of the material when the forage is dry should reduce the amount of fertilizer sticking to foliage where it is more susceptible to volatilization loss than if it makes contact with the soil.
Ammonium nitrate is the primary non-urea source used in hay and pasture systems. It contains 34 pounds of nitrogen per 100 pounds of fertilizer material – 34 percent nitrogen. It is usually the most expensive of the commonly used nitrogen sources. Availability may become a problem in some areas due to regulatory issues and storage problems.
For more information, contact the UT-TSU Extension Office in Wilson County at 615-444-9584. You can also find us on Facebook or visit extension.tennessee.edu/wilson. Ruth Correll, UT Extension-TSU Cooperative Extension agent in Wilson County, may be reached at 615-444-9584 or [email protected]