Agriculture Reference
In-Depth Information
Mineralization and Immobilization of P and S
4.4
Phosphorus
Soil P is normally the major reservoir of P in the P cycle between the soil and
plants. The soil P content ranges from 500 to 2,500 kg P/ha, of which 30-85%
may exist as organic P with the remainder in inorganic forms. Very little of this
P is immediately available to plants. As in the case of N, mineralization of organic
P occurs when microorganisms oxidize C substrates to obtain energy for growth.
Some P-rich substrates, such as nucleic acids and nucleotides, are rapidly miner-
alized to orthophosphate. But when the C:P ratio of the substrate exceeds 60-100,
net immobilization occurs because there is insufficient P to satisfy the microor-
ganisms, especially bacteria, which have a high P requirement per unit weight.
Thus, in ecosystems with only small P inputs, such as natural grasslands, the soil
organisms are highly competitive with higher plants for P. Bacterial residues rich
in P comprise mainly insoluble Ca, Fe, and Al salts of inositol hexaphosphate,
called
phytates
. These are strongly adsorbed to soil particles and hence are pro-
tected from further decomposition.
The main form of P in the soil solution that is available to plants is or-
thophosphate: the ions H
2
PO
4
and HPO
4
2
, whose proportions are controlled
by the reversible equilibrium
H
2
PO
4
4.4.1
HPO
4
2
H
(4.5)
At pH 7.2, the proportions of the monovalent and divalent orthophosphate ions
are equal. If the pH falls to 5, the ratio of H
2
PO
4
to HPO
4
2
is 100:1, whereas
at pH 9 the ratio of H
2
PO
4
to HPO
4
2
is 1:100. Mycorrhizae can also influ-
ence plant P uptake (section 4.7.3.2).
Sulfur
Soil S is derived originally from sulfide minerals in rocks that are oxidized to SO
4
-
S on weathering. This sulfate is absorbed by plants and returned to the soil in or-
ganic residues. Normally, therefore, soil S occurs mainly in an organic form
amounting to 200-2,000 kg S/ha. Organic S is mineralized to SO
4
2
ions dur-
ing microbial decomposition. The C:S ratio of well-humified organic matter is
50-150, which sets the approximate critical value for net mineralization of S.
Many vineyard soils are clean-cultivated. This inevitably results in lower or-
ganic matter contents than these soils would have in their natural state, so the
amount of S (as well as N and P) released by organic matter decomposition is
small. However, elemental S is regularly applied as a fungicidal “wettable pow-
der,” and any S residue that reaches the soil will be oxidized to SO
4
2
by
Thiobacil-
lus
bacteria (section 5.4.2.2). Although SO
4
-S is adsorbed by sesquioxides to some
extent, much will leach and tend to accumulate in the subsoil, as, for example, in
the Lateritic Podzolic Soils under vines in the Margaret River region of Western
Australia.
4.4.2
Labile and Nonlabile Pools
H
2
PO
4
, HPO
4
2
, and SO
4
2
ions are adsorbed by mineral particles from the
soil solution. Phosphate ions are much more strongly adsorbed than sulfate, for
4.4.3
