Agriculture Reference
In-Depth Information
Forms of S Fertilizers
Sulfur is the forgotten nutrient in vineyards, mainly because it is a constituent of
several fertilizers, such as SSP (11% S), potassium sulfate (K
2
SO
4
, 18% S), and
gypsum (CaSO
4
.2H
2
O, 19% S). A significant amount of sulfur can come from
atmospheric input. In addition, elemental S is used as a fungicide. Although up
to 28 kg/ha of “wettable S” may be applied in one year, much of this volatilizes.
The residue is washed into the soil where it is slowly oxidized by specialist
Thiobacillus
bacteria, according to the reaction
S
3/2O
2
H
2
O
2H
SO
4
2
(5.8)
Equation 5.8 shows that the oxidation of 32 kg S/ha produces 2 moles H
ion/ha. Thus, S applied as a fungicide can cause soil acidification, which must be
taken into account in managing the pH of vineyard soils.
5.4.2.2
Leaching and Residual Effects
Because of P fixation in the soil, P losses by leaching are usually small (0.1-0.2
kg/ha/yr). Exceptions occur with very sandy soils under high rainfall or irrigation,
or with soils that have been heavily fertilized for many years, where losses can
amount to 3-5 kg P/ha/yr. Sulfate, on the other hand, is not strongly adsorbed
and so is leached from the profile, except in dry climates when it may accumu-
late in the subsoil as gypsum.
Only a small fraction of fertilizer P is absorbed by vines during the year of
application. Much of the remainder is retained as fertilizer reaction products, which
become less soluble with time, and the rest is adsorbed to clays and sesquioxides
(section 4.5.4). A rough guideline is that two thirds of the soluble P remains avail-
able after one year, one-third after two years, one-sixth after three years, and none
after that. The residual effect of S is small unless it is applied as elemental S.
5.4.2.3
Calcium, Magnesium, and Potassium
Ca is mainly a “structural” element in cell walls and does not recycle in the vine.
Mg is required mainly for chlorophyll and as an enzyme cofactor. K occurs mainly
in ionic form and is the most mobile of the three elements. Fifty percent or more
of the vine K can be redistributed to the fruit by harvest time. Sucrose accumu-
lation and K transport appear to be coupled in the ripening berries. High K con-
centration in the berries is associated with high juice pH, which in turn leads to
high wine pH. The influence of berry K concentration on wine quality is dis-
cussed in section 9.8.
About 2.5-4, 0.5-0.6, and 0.1-0.2 kg of K, Ca, and Mg, respectively, is re-
moved per tonne of fruit. The higher K values occur in the hot Sunraysia and
Riverland regions of southeastern Australia, where the deep sandy soils are rela-
tively high in K.
Ca, Mg, and K are released by the weathering of feldspars, micas, and sec-
ondary minerals such as calcite, magnesite, and the micaceous clays. The presence
of these minerals depends on the soil's parent material and its age. For example,
Ca is abundant in soils formed on limestone and chalk, Mg is high in “green-
sands” (high in glauconite) and serpentinite, and K is high in soils formed on mica
schists. Soils formed on basic igneous rocks (basalts, dolerites, and gabbros) are
usually high in Ca and Mg. The pool of exchangeable Ca
2
, Mg
2
, and K
pro-
5.4.3
