Agriculture Reference
In-Depth Information
reduced forms in strongly gleyed soils (section 1.3.3.2). Nevertheless, because
atmospheric inputs are small (except in heavily industrialized areas), cumulative
losses over time from very old soils that are sandy may lead to micronutrient de-
ficiencies, as in some of the coastal regions of southern and southwestern Aus-
tralia.
Correction of Soil Acidity
Soil acidification is a natural process, but it can be accelerated under viticulture,
especially when NH
4
-N fertilizers and S-based fungicides are used. If concentra-
tions of exchangeable Al
3
in acid soils exceed 1 mg/kg soil, root growth is in-
hibited.
To correct soil acidity, various liming materials based on CaCO
3
are used,
depending on their availability and price (table 5.9). The dissolution of CaCO
3
in soil is an alkali-producing reaction, according to the equation
5.5.3
CaCO
3
CO
2
H
2
O
Ca(HCO
3
)
2
(5.11)
which shows that dissolution depends on the partial pressure of CO
2
in the soil
air. At a partial pressure of 0.0365 kPa (atmospheric) and a Ca concentration in
solution of 1 mmol/L, the equilibrium pH attained in the presence of solid CaCO
3
is 8.4.
The amount of lime required to raise the pH to a desired value depends on
the soil's titratable or total acidity (section 4.6.3.3). This amount is called the soil's
lime requirement
, the measurement of which is discussed in box 5.6. Lime is best
applied during autumn so that it begins to dissolve in the moist soil during win-
ter while the vine is dormant.
Liming Materials and Their Neutralizing Value
Table 5.9
Material and
Neutralizing
NV of
Value (NV)
a
Chemical
Commercial
Composition
(%)
Grade (%)
Comment
Burnt lime, CaO
179
120
Reacts vigorously with water
Occurs as a very fine powder;
difficult to handle
More soluble than calcium
carbonate, ca. 11% Mg
NV depends on concentration of
impurities such as clay, silica,
sesquioxides
By-product of cement
manufacture; fine powder, often
pelletized
Hydrated lime, Ca(OH)
2
135
105
Dolomite, (Ca,Mg)CO
3
109
100-119
Limestone, CaCO
3
—
50-85
plus impurities
Kiln dust, CaCO
3
, CaO,
—
90-110
some K
a
Calculated on the basis of the pure material, relative to calcite, which is 100% CaCO
3
Source:
Data from Goldspink (1996) and Cass (1998)
