Geology Reference
In-Depth Information
high pH (see Section 5.2). In general, therefore, low pH results in a
lessening of the negative charge on the surfaces of soil particles.
pH has a big influence on the solubility of many important elements in
soil, and can have a big influence on the availability of plant nutrients
(Figure 11). Aluminium, which is not required by plants, is relatively
insoluble except under acid conditions when toxicity problems may
occur. Using gibbsite, Al(OH)
3
, which is a common Al mineral in many
soils, as an example, the effect of pH on the concentration of aluminium
in solution can be modelled.
Worked example 5.5 - effect of pH on concentration of aluminium in
soil solution
What is the concentration of Al
31
ions in soil solution at the pH
values calculated in Worked example 5.4 for soil solution at (a) pCO
2
¼
10
2.5
atm and (b) pSO
2
¼
10
7
atm?
Using the dissolution of gibbsite as a model,
Al(OH)
3
Al
31
þ
3OH
Solubility product,
K
SP
¼
5
10
33
"
(5.18)
½
Al
3
þ
½
OH
3
¼
5
10
33
ð
a
Þ
At pCO
2
¼
10
2
:
5
atm
pH
¼
5
:
2
;
pOH
¼
8
:
8
½
OH
¼
1
:
58
10
9
½
OH
3
¼
3
:
94
10
27
½
Al
3
þ
¼ð
5
10
33
Þ=ð
3
:
94
10
27
Þ
¼
1
:
27
10
6
molL
1
ð
b
Þ
At pSO
2
¼
10
7
atm
pH
¼
4
:
4
;
pOH
¼
9
:
6
½
OH
¼
2
:
51
10
10
½
OH
3
¼
1
:
58
10
29
½
Al
3
þ
¼ð
5
10
33
Þ=ð
1
:
58
10
29
Þ
¼
3
:
2
10
4
molL
1
:
