Nitrogen Application
What, when, where and how you apply nitrogen matters.
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| Figure 5. Generalized 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 application of preplant N. 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 5 and Table
7). 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 8 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 expensive on a price per pound of N basis. 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 optimize your N dollar.
