YORK UNIVERSITY
Lab 2-Cation Exchange Capacity
GEOG 3600
Paloma D'Silva
3/6/2013
By knowing the CEC of the clay-size fraction in soil, it is possible to find out clay minerals present. In this
report, the CEC per 100 g clay has been compared with the % C/100 g clay in order to find out what clay
minerals might be present in the soil.
1
Lab 2-Cation Exchange Capacity
1.0 Introduction
The CEC of a soil, or its cation exchange capacity, is the maximum ability of a soil to hold cations
available for exchange, and is expressed as milliequivalents/100 g soil. Various clay-size minerals
have CECs that fall within certain limits, and by knowing the CEC of the clay-sized fraction in the soil
it is possible to estimate the clay minerals present. CEC varies with clay mineral species, in addition
to organic matter content, and the higher the organic matter content of the soil, the higher CEC will
be (Birkeland, 1999). CEC is used to estimate the clay minerals present in the soil sample by making
use of soils' property to exchange cations. The soils data has been given for %C and CEC for 100g soil
and must be recalculated with respect to 100 g clay. The CEC has been taken as a <2mm fraction,
and recalculated with respect to the clay percentage in the soil samples.
2.0 Methods
In order to calculate the CEC and %C with respect to clay, it is done for 100g of soil. The soil
amount of 100 g clay is divided by the total clay percentage, and then multiplied by the
percentage of carbon to get %C per 100 g clay. Similarly, for CEC/100 g clay, the amount of 100
is divided by the clay percentage and then multiplied by CEC for the <2mm fraction per 100 g
soil, which has been expressed as a percentage in the soil profiles. The calculations have been
graphed for three soil profiles, Profile 44, Profile 99, and Profile 86. All data used here has been
obtained from pages 101-103 of GEOG 3600 course kit (Mahaney, 2013).
3.0 Results
3.1 Clay Mineral Calculations
Both CEC and %C (which represents organic matter content of the soil) have been calculated
as a fraction of 100 g and have been graphed in the corresponding soil profiles.
3.1.1 Profile 44
Table 1
Profile
44
Raw
Raw %C CEC
%clay
CEC/100g %C/100g
3.74
38.90
41
94.88
9.12
1.18
37.60
46.2
81.39
2.55
0.51
41.10
51.3
80.12
0.99
0.18
45.60
59.5
76.64
0.30
0.13
45.60
60.1
75.87
0.22
0.1
46.60
59
78.98
0.17
0.1
47.20
56.8
83.10
0.18
2
0.06
47.30
56
84.46
0.11
Table 1 shows the data for profile 44. The last two columns have been graphed in Figure 1.
Figure 1
According to Figure 1, the trendline hits the 0%C value at 80 meq/100g CEC. This is the xintercept of this graph, and the CEC value most closely corresponds to a clay mineral with a CEC
of 80 meq/100g.
3.1.2 Profile 86
Profile 86 has been taken from Tehama County in California.
Table 2
Profile
86
Raw %C
0.36
0.18
0.14
0.35
0.18
0.05
Raw
CEC
%clay
CEC/100g %C/100g
7.50
9.60
78.13
3.75
8.00
10.90
73.39
1.65
8.60
12.20
70.49
1.15
32.80
55.50
59.10
0.63
33.00
26.00
126.92
0.69
28.10
26.00
108.08
0.19
Figure 2
3
The last two columns in Table 2 have been graphed in Figure 2.
According to Figure 2, the trendline hits the 0%C value at approximately two values, 50 meq/100
g CEC and 110 meq/100 g. These are the x-intercepts of this graph, and the CEC value most
closely corresponds to a clay mineral with a CEC of 50 meq/100 g or 110 meq/100 g. This
observation could mean that the clay mineralogy changes with depth due to changing chemical
environment with depth, and that it is possible that there is more than one clay mineral in the
soil profile.
3.1.3 Profile 99
Profile 99 has been taken from Puerto Rico.
Table 3
Profile
99
Raw
Raw %C CEC
%clay
CEC/100g %C/100g
2.72
24.30
72.40
33.56
3.76
1.42
21.70
76.10
28.52
1.87
0.66
18.60
67.80
27.43
0.97
0.24
15.50
59.00
26.27
0.41
0.22
16.60
59.70
27.81
0.37
0.26
15.90
57.60
27.60
0.45
4
0.16
0.21
0.06
13.20
13.80
16.40
38.10
34.90
32.40
34.65
39.54
50.62
0.42
0.60
0.19
Figure 3
Profile 99
4.00
%C/100 g clay
3.50
3.00
2.50
2.00
1.50
1.00
0.50
0.00
0.00
10.00
20.00
30.00
40.00
50.00
60.00
CEC/100g clay
The last two columns in Table 3 have been graphed in Figure 3. According to Figure 3, the
trendline hits the 0%C value at 25 meq/100g CEC, though there are a few outliers. This is the xintercept of this graph, and the CEC value most closely corresponds to a clay mineral with a CEC
of 25 meq/100g. The outliers could suggest variations in the clay mineralogy present in the soil.
4.0 Analysis
As mentioned previously, in order to analyze the CEC of the clay-sized fraction present in the soil for
100 g of clay, where the projected line hits 0%C, this provides an approximation of the clay-sized
fraction. An attempt has been made to estimate the clay minerals present in the data. Please note
that the CEC values given are at a pH of 7. An estimation of the clay mineralogy has been conducted
using a reference guide(Birkeland, 1999), and the clay minerals differ according to their CEC values.
For Profile 44, there is a CEC of 80 meq/100 g of clay. According to the reference guide used, this
corresponds to smectite, and it is possible that the prevalent clay mineral here is smectite. Smectite
is found in younger soils.
For Profile 86, there is a CEC of 50 meq/ 100 g clay and 110 meq/100 g clay. This indicates that there
are two types of clay minerals found in this profile, and according to the reference guide used, the
clay minerals could be halloysite, allophane, smectite, or vermicullite. It is possible that there are
equal contributions from halloysite and allophane, or smectite and vermicullite.
5
For Profile 99, there is a CEC of approximately 25 meq/100 g clay. This value corresponds to the
palygorskite group, illite, chlorite, sepiolite, and allophane. Based on the other information in this
soil profile, it is possible that the prevalent clay mineral in this soil profile is of the chlorite and illite
group, containing equal contributions of each.
5.0 References
Birkeland, Peter W. Soils and Geomorphology: Third Edition. New York, New York: Oxford University
Press, 1999.
Mahaney, W.C. GEOG 3360 Course Kit. Toronto, Ontario: York University, 2013.