Agriculture Reference
In-Depth Information
Potential Losses from Nitrogen Fertilizers
The uptake of fertilizer N applied in a single season is usually less than 50%.
Nitrate from Ca(NO
3
)
2
and NH
4
NO
3
is most readily absorbed. Although some
of the NH
4
-N from NH
4
NO
3
and urea is immobilized by microorganisms,
most is oxidized to NO
3
−
.MuchoftheNO
3
-N that is not taken up by the
vines or a mid-row cover crop is lost by leaching or denitrification, the latter
occurring when a soil becomes waterlogged (see “What Causes Poor Drainage?”
chapter 4).
Not only is N lost as NO
3
−
through leaching, but nitrification followed
by leaching also acidifies the soil (see box 3.2). Potentially the most acidifying
N fertilizers are those in which all N is present as NH
4
-N, such as in urea and
(NH
4
)
2
SO
4
. Less acidifying is NH
4
NO
3
, whereas Ca(NO
3
)
2
and KNO
3
have
no acidifying effect at all. This is one reason why Ca(NO
3
)
2
is favored as an N
fertilizer in vineyards on acid to neutral soils, especially when applied by drip
irrigation. Potassium nitrate is not recommended if the soil already supplies
adequateK.
Urea applied to moist soil rapidly hydrolyzes to produce NH
4
+
ions and
NH
3
gas that is lost by volatilization. High temperatures and wind accelerate
NH
3
loss, and, in extreme cases, up to 50% of the N in surface-applied urea
can volatilize. Granules of urea or NH
4
NO
3
are best placed a few centimeters
into the soil or dissolved in irrigation water to avoid NH
3
loss. However, if
too much irrigation is applied, the dissolved urea may leach to greater depths
before it hydrolyzes to NH
4
+
ions, in which case the acidity that develops sub-
sequentlythroughnitriicationisdiiculttocorrectbecauseitisinaccessibleto
surface-applied lime.
Controlled-release fertilizer or slow-release fertilizer can be used to
reduce N losses. Controlled-release fertilizers, such as sulfur-coated urea
(SCU,31%-38%N),polymer-coatedSCU(<30%N),andureaproducts
with a urease inhibitor (46% N), are synthetic compounds for which nutrient
release is slow and well controlled. The slow-release fertilizers are synthetic
or natural products for which nutrient release is slow but not necessarily well
controlled, because it depends on microbial action in the soil. Examples are
blood and bone (5%-6% N) and hoof and horn meal (7%-16% N). Both
controlled-release fertilizers and slow-release fertilizers are sparingly soluble
materials that do not produce the localized high concentrations of NH
4
+
and
NO
3
−
that predispose to N loss.
When to Apply Nitrogen Fertilizer
The timing and amounts of N influence vine vigor and canopy development, bud
fruitfulness, juice N, and N storage in woody tissues. Figure 3.13 shows the peri-
ods of N uptake or remobilization within the vine in relation to its stage of growth.
