Nitrogen Application

What, when, where and how you apply nitrogen all matter.

Figure 3. Nitrogen uptake pattern across growth stages.

Well-timed and placed nitrogen (N) fertilizer applications can greatly enhance plant uptake of nitrogen and maximize return when you’re trying control input costs. The greatest N use efficiency occurs when N fertilizer is applied in increments to match crop needs. If possible, reduce or eliminate applying N preplant. When this is not an option, consider applying an ammonium N form, such as urea or anhydrous ammonia, because the ammonium ion (NH₄+) is not subject to immediate leaching. Nitrate (NO₃) forms of N fertilizer are readily available to crops, but are subject to leaching losses. Although transformation of NH₄ to NO₃ under warm, moist soil conditions occurs rapidly, applying ammonium forms early in the season may reduce leaching losses. Slow release N fertilizers may also be feasible this year, particularly for high value crops. Immediately incorporate all surface applied fertilizers to reduce runoff and volatilization.

Fertilizer applications should be timed to coincide as closely as possible to the period of maximum N uptake which is during rapid vegetative growth (see Figure 3 and Table 8). A starter fertilizer, followed by sidedress application or fertigation improves crop N uptake efficiency and is a good strategy this year. Application of N through high efficiency irrigation systems such as center pivot or surge systems during rapid growth maximizes crop N uptake. It is not recommended that nitrogen be applied through low efficiency furrow or flood systems due to runoff and deep percolation losses. Waiting until the crop is well established before applying large amounts of N also allows you to more accurately determine the crop yield potential. Poor stands, weed control, and below average precipitation are good reasons to adjust N rates downward at sidedress time. Conversely, exceptional conditions warrant increased N at sidedress.

Another thing producers must keep in mind when making fertilizer choices is how much you are actually paying for the element nitrogen. Often producers will price fertilizers by the ton rather than by the pound of actual N. This is unfortunate because price per ton can be misleading. Table 9 presents a comparison of cost of nitrogen fertilizer per ton and per pound of N. Notice that 28% UAN has the cheapest price per ton, but is among the most expensive sources of N for actual price per pound. Conversely, anhydrous ammonia is the most expensive N fertilizer in price per ton, but is the cheapest source for actual price per pound of N.

Nitrogen is not a stable element in soil and some portion of your fertilizer will be lost to leaching and other transformations. Application timing, technique and fertilizer form all can help reduce these loses and keep your N dollar doing its job, producing yield.