Agriculture Reference
In-Depth Information
Box 5.6
Measuring a Soil's Lime Requirement
The
lime requirement
is the quantity of ground limestone or chalk (t/ha to a
depth of 15 cm) required to raise the soil pH to a desired value. In Australia, a pH
of 6.5 (1:5 in H
2
O) is recommended for vineyards (Goldspink 1996). However, in
parts of Beaujolais and Languedoc-Rousillon in France, grapevines have been
grown successfully on acid soils (as low as pH 4) for many years. At minimum,
sufficient lime should be applied to hydrolyze the exchangeable Al
3
(to
1
mg/kg), which is normally achieved at pH 5.5. Gypsum does not have a liming
effect, but the Ca
2
supplied competes with Al
3
during ion uptake and hence
ameliorates the adverse effects of Al.
Methods of measuring the lime requirement range from equilibrating a soil
sample with a single buffer solution to titrating the soil with Ca(OH)
2
. Details of
laboratory methods are given in Rayment and Higginson (1992). Soils with a small
pH buffering capacity, such as sandy loams poor in organic matter, have a smaller
lime requirement than soils with a high pH buffering capacity, such as clays and
organic-rich loams. The lime requirement ranges from 1-2 t CaCO
3
/ha for sandy
loams to 4-5 t CaCO
3
/ha for clay soils.
Laboratory analysis is also necessary to measure the
neutralizing value
(NV ) of
the liming material. The standard in Australia is pure CaCO
3
. The NV of various
liming materials is given in table 5.9. Another important property is the fineness of
the liming material, because the smaller the particle size, the faster the reaction
with the soil. A high-quality liming material should have an NV
75% with 80%
of the particles
0.6 mm diameter. To be most effective, lime should be cultivated
into the surface soil.
Reactions Affecting Nutrient Availability in Waterlogged Soil
5.6
Anaerobic Processes
When the soil pore space fills with water, the soil becomes waterlogged. Soil aer-
ation is then inadequate to sustain aerobic respiration (section 3.4), and mi-
croorganisms capable of respiring anaerobically begin to multiply. Carbohydrates
are fermented to organic acids such as acetic and butyric acids, which are subse-
quently metabolized to the gases CO
2
and CH
4
, with some ethylene (C
2
H
4
) and
H
2
also produced. Fermentation releases much less energy for microbial growth
than aerobic respiration. Although low concentrations of C
2
H
4
may stimulate root
elongation, concentrations
1 mg/L in the gas phase may inhibit root growth.
In addition to their direct effect on root metabolism and growth, anaerobic
conditions predispose to a chain of reactions that can have undesirable effects.
These are called
redox reactions
because they involve the transfer of electrons (e
)
produced by respiration to acceptors other than O
2
. The first of these reactions—
NO
3
reduction, otherwise called
denitrification
—can occur in partially water-
logged soil, that is, when the macropores between aggregates still contain O
2
, but
the small pores inside the aggregates are anoxic (see fig. 3.7). The complete reac-
tion for denitrification is
2NO
3
12H
10e
N
2
(gas)
6H
2
O
5.6.1
(5.12)
