Geology Reference
In-Depth Information
HSO
3
þ
H
1
K
¼
1.7
10
2
H
2
SO
3
"
(5.13)
½
HSO
3
½
H
þ
½
H
2
SO
3
¼
1
:
7
10
2
mol L
1
½
H
þ
2
¼
1
:
7
10
2
molL
1
1molL
1
atm
1
10
7
atm
¼
1
:
7
10
9
½
H
þ
¼
4
:
12
10
5
log
½
H
þ
¼
4
:
4
pH
¼
4
:
4
Various soil processes also contribute to soil acidity. Decaying organic
matter releases a number of organic acids, as indeed do plant roots.
Nitrification, the microbial oxidation of ammonium ions to nitrate by
Nitrosomonas and Nitrobacter, occurs in slightly acidic to neutral soils
and releases H
1
ions:
2NH
4
þ
3O
2
2NO
2
þ
4H
1
þ
2H
2
O
-
(5.14)
2NO
2
þ
O
2
2NO
3
-
(5.15)
Sulfide oxidation, another microbially mediated process, also results in
the production of acidity:
2FeS
2
þ
2H
2
O
þ
7O
2
"
2FeSO
4
þ
2H
2
SO
4
(5.16)
4FeSO
4
þ
10H
2
O
þ
O
2
"
4Fe(OH)
3
þ
4H
2
SO
4
(5.17)
Note that in this case much of the acidity is due to the precipitation of
iron(III) oxide.
Leaching, the passage of water through the soil profile, is a process
that occurs in soils in areas of moderate to high rainfall, and which
results in the loss of exchangeable cations (such as Ca
21
,Mg
21
,K
1
, and
Na
1
- see Section 5.5). Unless the rate of weathering of soil minerals is
suciently rapid to replace these losses, the exchange sites will become
occupied by H
1
and Al
31
, which are acidic cations.
5.4.3 Soil Alkalinity (see also Section 3.2.4)
Alkaline soils (pH 4 7) contain solid phase carbonate, and bicarbonate
is the dominant anion in solution. Calcium and magnesium carbonates
