Agriculture Reference
In-Depth Information
Forms of Soluble N Fertilizer Used in Vineyards
Table 5.6
Compound
Formula
N Content (%)
Comments
Potassium nitrate
Calcium nitrate
KNO
3
Ca(NO
3
)
2
13.4
15.5
KNO
3
and Ca(NO
3
)
2
are very
soluble, suitable for fertigation;
nonacidifying, but more expensive
per kg N than other forms
Supplies N and S in soluble forms;
acidifying
NO
3
is immediately available; NH
4
is adsorbed by soil and oxidized to
NO
3
Very soluble, high N content; prone
to volatilization if not washed into
the soil; acidifying
Has the advantages and disadvantages
of urea and NH
4
NO
3
Ammonium sulfate
(NH
4
)
2
SO
4
21
.0
Ammonium nitrate
(“Nitram”)
NH
4
NO
3
34
.0
Urea
(NH
2
)
2
CO
46
.0
Urea ammonium
nitrate solution
(NH
2
)
2
CO,
NH
4
NO
3
in water
30-32
and horn meal). Short-term plant response to these fertilizers is usually inferior,
per kg of N, compared with the soluble forms; but they are useful on sandy soils,
especially if N fertilizer is applied during winter.
The amount of N fertilizer required depends primarily on the balance among
N inputs, N removal in prunings and in the fruit at harvest, and any losses. Before
discussing how to estimate amounts, we must consider the loss pathways for N.
Nitrification, Leaching, and Acidification
NH
4
ions from N fertilizer, or from net mineralization of organic N, will ni-
trify in the soil (section 4.3). The overall reaction is described by equation 4.4,
which shows that 2 moles of H
ion are produced for every mole of NH
4
ion
oxidized. Thus, adding NH
4
-N fertilizers has the potential to acidify the soil. If
the NO
3
formed is absorbed by the vine, there is some compensatory release of
HCO
3
(which breaks down to CO
2
and OH
ions) by the roots during up-
take. Moreover, if there is recycling of plant N to the soil through leaf fall and
decomposition, there is no net acidification from this component. But less than
50% of fertilizer N may be taken up by the vines. Of the NO
3
-N not taken up,
some is immobilized, but much is lost by leaching or denitrification. The NO
3
leached is accompanied by cations, such as Ca
2
, because the cations are displaced
by H
, as shown here:
Ca
2
-clay
2H
5.4.1.2
2H
-clay
Ca
2
(5.3)
The H-clay becomes an Al-clay (section 4.6.3.1). Potentially, the most acid-
ifying N fertilizers are those in which all the N is present as NH
4
-N, such as urea
and (NH
4
)
2
SO
4
. Less acidifying is NH
4
NO
3
, and Ca(NO
3
)
2
and KNO
3
have no
acidifying effect at all. This is one of the reasons why Ca(NO
3
)
2
is favored as an
N fertilizer in vineyards, especially when applied by drip irrigation. KNO
3
is not
favored if the soil already supplies adequate K.
