Nitrogen Ferilizer Management
Nitrogen is a nutrient easily lost from soil through several pathways, as already discussed. Consequently, plants use nitrogen most efficiently if the producer applies it as closely as possible to the time of crop uptake. Ideally, nitrogen might be applied several times during a growing season. Center pivot irrigation systems equipped for fertigation and high clearance applicators are two methods to accomplish multiple nitrogen applications. Fertigation systems also facilitate the use of a chlorophyll meter to detect nitrogen deficiency and allow for nitrogen applications according to crop demand. Current recommendations suggest applying supplemental nitrogen if the chlorophyll meter reading from a field is less than 95 percent of an adequately fertilized check strip, up to 20 days after silking. Rates should not exceed 40 pounds of nitrogen per acre. A sidedress nitrogen application also allows for more efficient fertilizer use since the producer applies nitrogen close to the period of maximum nitrogen uptake for corn and sorghum. Nitrogen application prior to or at planting is still more efficient than fall application for row crops such as corn and grain sorghum. Fall application may still be viable on some soils for row crops. Anhydrous ammonia should only be applied in the fall (because it initially is not leachable), if soils are fine-textured and when the soil temperature is 50oF, on average, for a week or longer. With either fall or spring preplant application, nitrification inhibitors, such as N-Serve® or DCD, help reduce the potential for leaching or denitrification losses of nitrogen. For nitrogen application to winter wheat, late winter or early spring top dress application allows the producer to assess moisture status and crop condition before determining the appropriate nitrogen rate.
Crops use nitrogen more efficiently when it is placed beneath the soil surface. Broadcasting nitrogen on the soil surface increases the likelihood that some nitrogen will be lost due to ammonia volatilization or runoff. This is one reason why anhydrous ammonia, which must be injected, sometimes appears to be a better nitrogen source than urea or urea ammonium nitrate (UAN) solution, which can be applied on the soil surface. In general, as long as nitrogen fertilizers are correctly applied, all are agronomically equal. If the farmer must apply nitrogen fertilizers to the soil surface, he can increase efficiency by banding, which concentrates the fertilizer and reduces soil/fertilizer contact. Applying water through a sprinkler irrigation application is another efficient method, as long as application rates are not excessive.
The primary nitrogen fertilizers available in Nebraska are anhydrous ammonia (82 percent nitrogen), urea (44-46 percent nitrogen), UAN solution (28-32 percent nitrogen), ammonium nitrate (33-35 percent nitrogen), and ammonium sulfate (21 percent nitrogen). Other fertilizers can contain significant amounts of nitrogen, but they are used primarily as sources of other nutrients. All of the above are effective fertilizers when properly applied. Anhydrous ammonia is historically the least expensive nitrogen fertilizer, but it requires injection into the soil, which is a more expensive application method than broadcasting or surface banding. Tillage, irrigation and rainfall soon after application reduce the potential for significant ammonia loss from urea fertilizers. A recent management option for urea fertilizers is the urease inhibitor Agrotain®. This material contains the active ingredient N-(n-butyl) thiophosphoric triamide (NBPT), which inhibits the function of the urease enzyme (responsible for breaking urea down into ammonium and potentially ammonia) for up to several weeks, depending on temperature. This delay in decomposition of urea can increase the chances of rain or tillage moving the urea into the soil where it is protected from volatile loss. Using a urease inhibitor reduces the risk of applying urea fertilizers on the surface in minimum-tillage — high residue conditions which otherwise have considerable potential for ammonia loss. Urease inhibitors, like nitrification inhibitors, will not guarantee a yield increase every year, but they can protect against yield reductions in years when climatic conditions are conducive to nitrogen loss.