Agriculture Reference
In-Depth Information
7
6
5
pH
4
3
0
20
40
60
80
100
120
pH titration curve for
Figure 4.10
Alkali added (cmol OH
-
/kg clay)
an Al-vermiculite.
and build into polymeric units of six or more Al atoms. The residual H
ions as-
sociated with these hydroxy-Al ions add to the soil's capacity to neutralize OH
ions. This acidity, together with the soil's exchangeable acidity (Al
3
H
), is
called the
titratable acidity
(or total acidity).
The ability of a soil to neutralize OH
ions is a measure of its
pH buffering
capacity
. The release of H
through Al hydrolysis and polymerization augments
the soil's pH buffering capacity, with neutralization proceeding until Al(OH)
3
precipitates. Similarly, dissociation of H
ions from organic matter adds to the
buffering capacity. The gradual neutralization of an acid clay by slow addition of
an alkaline solution creates a pH titration curve, an example of which is shown
in figure 4.10. The flatter the slope of the titration curve, the greater is the buffer-
ing capacity of the clay. The strongest acid groups are neutralized first, followed
by progressively weaker groups. Some “weak” acidity due to H
ion release from
hydroxy-Al polymers continues to be neutralized at pH
7.5. The quantity of
acid neutralized when a soil is titrated with 0.1M BaCl
2
/triethanolamine solution
at pH 8.2 is a measure of its titratable acidity (cmol H
/kg), which is invariably
greater than the soil's exchangeable acidity. In practice, titration of a soil to a spe-
cific pH is used to estimate a soil's
lime requirement
(section 5.5.3).
The Supply of Ions to Plant Roots
4.7
Transport Processes
Ions move to a plant root by
mass flow
and
diffusion
. Mass flow occurs when the
plant is transpiring and absorbing soil water that contains dissolved ions. The ions
are swept along in the “transpiration stream” to the root surface. A similar mass
flow (sometimes called convective flow) occurs when ions in the soil solution are
leached downward by percolating water. Ion diffusion, on the other hand, is a
molecular process that occurs in response to a concentration gradient (as discussed
in box 3.7) and is independent of the flow of water. Compared to mass flow, dif-
fusion is effective only over relatively short distances.
4.7.1