Agriculture Reference
In-Depth Information
Clay mineral
SiO
2
+
2H
2
O
Al
3+
Figure 4.9
Weathering of a clay
mineral under acid
conditions to form an Al-
clay (White 1997).
Reproduced with
permission of Blackwell
Science Ltd.
H
+
+ HCO
3
-
Hydroxy
Al ions
H
+
K
+
Si(OH)
4
(leached)
Primary processes
Secondary processes
High concentrations of exchangeable Al
3
have an adverse effect on the
growth of grapevine roots (section 5.5.3).
4.6.3.2
Base Saturation
In calcareous soils, the sum of the exchangeable cations, expressed as
(Ca, Mg,
K, Na) (meaning “sum of”), is invariably equal to the effective
CEC
because any
deficit of exchangeable cations is made up by Ca
2
ions dissolved from CaCO
3
.
In noncalcareous soils,
(Ca, Mg, K, Na) is usually less than the effective
CEC
,
the difference being made up by the exchangeable acidity, that is,
Effective CEC
(Ca, Mg, K, Na)
(Al
3
H
)
(4.15)
The ratio
(Ca, Mg, K, Na)
100
CEC
is commonly called the
percent base saturation
. Historically these cations have been
called “basic” because they are the cations of strong bases in water, such as NaOH,
KOH, Mg(OH)
2
, and Ca(OH)
2
. The term is useful to distinguish these cations
from the “acidic” cations, such as Fe
3
and especially Al
3
, which hydrolyze in
water to release H
ions in the pH range 4-6. For example, Al
3
in water is re-
ally a hexahydrated ion that hydrolyzes as follows:
Al(H
2
O)
6
3
H
2
O
[Al(OH)(H
2
O)
5
]
2
H
3
O
(4.16)
The ion on the right-hand side is called a hydroxy-aluminum ion. There are
equal concentrations of the divalent and trivalent Al ions at pH 5, which is the
pK value for this reversible reaction (see appendix 3).
4.6.3.3
Titratable Acidity and pH Buffering Capacity
As acid soils weather over many years, they produce appreciable amounts of ex-
changeable Al
3
, much of which hydrolyzes according to reaction 4.16. The
hydroxy-Al ions (AlOH)(H
2
O)
2
, tend to combine through bridging OH groups
