GEOG 3360 Lecture Notes - Soil Horizon, Palygorskite, Halloysite

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30 Mar 2013
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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. Organic matter contributes to soil CEC. 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 %C
Raw
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
0.06
47.30
56
84.46
0.11
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1
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 x-
intercept 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
Raw
CEC
%clay
CEC/100g
%C/100g
0.36
7.50
9.60
78.13
3.75
0.18
8.00
10.90
73.39
1.65
0.14
8.60
12.20
70.49
1.15
0.35
32.80
55.50
59.10
0.63
0.18
33.00
26.00
126.92
0.69
0.05
28.10
26.00
108.08
0.19
Figure 2
The last two columns in Table 2 have been graphed in Figure 2.
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Document Summary

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. 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. 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.

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