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
 
 
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