Suppose a soil contained 2% organic matter and 16% clay. The soil CEC may be estimated to be around (2 x 2) + (16 x 0.5) = 12 meq/100 g.

Rates of Cation Exchange

The rates of exchange depend on the clay type. Kaolinite does not swell in water and there is no inner-layer access. Exchange reactions in kaolinites are almost instantaneous because they occur only on the outer surface. Illites swell slightly. Exchange reactions in illites can take several hours because a small part of the exchange occurs between inner crystal unit layers, to which cation

* Clay is much denser than humic matter. An equal weight of clay represents many fewer milliequivalents. Copyright © 2000 CRC Press, LLC

diffusion is slow. Montmorillonite expands considerably and most of the exposed surfaces are on the inner layers. Montmorillonites take still longer to reach ion-exchange equilibrium.

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