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GEOG 3360 (2)
Lecture

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Department
Geography
Course
GEOG 3360
Professor
Bill Mahaney
Semester
Winter

Description
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 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 0.06 47.30 56 84.46 0.11 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 Raw %C 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. 2 According to Figure 2, the trendline hits the 0%C value at approximate
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