Geology Reference
In-Depth Information
but at most soil pHs the negative charge contributing to the CEC is
small.
In a soil of between pH 6 and 7, about 80% of the cation-exchange
sites hold Ca
21
, about 15%Mg
21
,K
1
, and Na
1
account for up to 5%,
with small amounts of NH
4
1
and H
1
. As the pH falls, Al
31
,Mn
21
, and
H
1
become relatively more important.
The % base saturation is a measure of the amount of the CEC of a soil
holding the major cations (also called exchangeable bases) Ca
21
,Mg
21
,
K
1
, and Na
1
.
% base saturation
¼
P
Ca
2
þ
;
Mg
2
þ
;
K
þ
;
Na
þ
CEC
100
ð
5
:
40
Þ
where the units of exchangeable cations and CEC are cmol
c
kg
1
.
% base saturation is high (approximately 80%) in fertile soils with
basic minerals in the parent material and low (
o
30%) in highly leached
acidic soils.
Worked example 5.9 - cation-exchange capacity and % base saturation
One method to measure the CEC of a soil is to leach a column
containing a known weight of soil with a solution of a salt so that the
exchange sites are saturated with the cation (index ion). After washing
out the excess salt solution, a different salt solution is leached through
the column and the exchangeable ions displaced, collected in a known
volume and their concentration measured.
10 g soil was leached with 200 cm
3
of 1 mol L
1
CH
3
COOK solution
in order to saturate the exchange sites with K
1
ions. After washing
out the excess CH
3
COOK solution, 200 cm
3
of 1 mol L
1
CH
3
COONH
4
solution was passed through to displace the K
1
ions
by NH
4
1
. The solution that leached through the soil was made up to
250 cm
3
in a volumetric flask. Ten centimetre cube of this solution
was diluted to 100 cm
3
and the K
1
concentration in the diluted
solution measured by flame photometry. If the K
1
concentration in
this solution is 41 mgcm
3
, what is the CEC of the soil in cmol
c
kg
1
?
Concentration of K
þ
in the diluted solution
¼
41 mgcm
3
¼
410 mgcm
3
in the original 250 cm
3
of solution
:
