Agriculture Reference
In-Depth Information
Box 3.2 Mineralization, Nitrification, and Their Effects on Soil pH
The first step in mineralization of organic N—the production of NH
4
+
ions—is carried out by a range of heterotrophic microorganisms and is called
ammonification. (Table 5.3 in chapter 5 explains the differences between
heterotrophic and autotrophic organisms). Because OH
−
ions are also produced to
electrically balance the NH
4
+
ions, this step creates alkalinity.
Ammonium ions are taken up by vine roots, but much of the soil NH
4
-N
is consumed by specialist autotrophic bacteria that derive energy for growth by
oxidizing NH
4
+
first to nitrite (NO
2
−
) and then to nitrate (NO
3
−
). This two-step
process, carried out by various species of
Nitrosomonas
and
Nitrobacter
bacteria,
respectively, is called nitrification and produces 2H
+
ions for every NH
4
+
oxidized.
The net outcome of ammonification followed by nitrification is that one mole each
of NO
3
−
and H
+
are produced for every mole of organic N that is oxidized (moles
and moles of charge are explained in box 3.4).
Although nitrification is potentially acidifying, this potential is not realized if
all the NO
3
−
produced is taken up by plant roots, releasing HCO
3
−
and OH
−
ions
in exchange, which cancel out the H
+
ions. However, NO
3
−
is readily leached from
the root zone by percolating water, in which case actual acidification takes place
because the leached NO
3
−
is accompanied by Ca
2+
and other exchangeable cations
that are displaced by the H
+
ions.
Soil acidification always occurs when an NH
4
-N fertilizer such as ammonium
sulfate ((NH
4
)
2
SO
4
) is added to soil, because when the NH
4
+
ions are oxidized,
two H
+
ions are released for every NO
3
−
ion produced, with no counterbalancing
OH
−
ions. The effect of this acidification is usually measured in terms of the “lime
(CaCO
3
)equivalent”requiredtoneutralizetheH
+
ions (i.e., to raise the soil pH).
Experienceshowsthatthepracticallimeequivalentof(NH
4
)
2
SO
4
is about 5.4 kg
pure CaCO
3
perkgNinthefertilizerandabout1.8:1forammoniumnitrate
(NH
4
NO
3
) fertilizer.
The effect of urea fertilizer ((NH
2
)
2
CO) on soil pH is similar to what occurs
when soil organic N compounds are oxidized, with one important difference. The
soil pH around a granule of urea initially increases to nine or more as ammonium
hydroxide (NH
4
OH) is formed from the urea. However, NH
4
OH is unstable
and breaks down to release NH
3
gas and H
+
ions, which neutralize the OH
−
ions.
heextentofanysubsequentacidiicationthendependsonhowmuchofthe
remaining NH
4
+
is oxidized to NO
3
−
and how much of this NO
3
−
is leached from
the soil.
The balance between mineralization and immobilization depends on whether
there is enough of an element in an organic substrate to satisfy the microorgan-
isms' demand for growth. The demand for N, S, and P is directly linked to the
demand for C, the main building material of microbial cells. For example, if the
C-to-N ratio in the substrate is small, there is likely to be surplus N that is released
as NH
4
+
, and thus net mineralization occurs. Conversely, if the C-to-N ratio is
