Cation Exchange and PBS

The Cation exchange capacity (CEC) & Percentage Base Saturation (PBS)

The cation exchange capacity (CEC) refers to the degree of charge expressed by a given volume of soil and its ability to hold and release positively charge ions. These are both acidic (Aluminium and Hydrogen) and basic, with the four essential macro elements of Calcium, Magnesium, Potassium and Sodium occupying more than 99% of the basic cation exchange capacity. The CEC is dependent on the clay type and % clay in the soil. It is effectively the sum of the total number of exchange sites on the clay content of the soil that can hold both acidic and basic cations and whilst some sites are interchangeable some are not. Thus the soil has both an acid and basic cation exchange capacity but from a fertility aspect we are concerned only with its basic capacity, the acidic component being unimportant in all but extreme situations.

This basic capacity is known as the effective cation exchange capacity and the distribution of the basic cations the percentage base saturation (PBS). It is the measure of the ratio (%) of the basic cations Calcium, Magnesium, Potassium and Sodium.

Whilst the measure of the effective cation exchange capacity and the PBS are extremely useful it is a moderately complex procedure to determine and requires specialised equipment. We can however approximate the cation exchange capacity if we know the dominant clay types [table 2a] and the relative fraction of the soil that is composed of clay.

Similarly if we assume the PBS has a normal distribution we can use the pH measurements to approximate the soils Calcium and Magnesium requirements. It is possible to measure the actual calcium and magnesium concentrations by titration however as with the CEC this is relatively complex procedure that should only be undertaken by a trained technician in a sound laboratory environment.

The soil solution is then maintained by a dynamic process with cations jumping in and out of solution as they exchange between different Cation sites on the clay crystal. In this way a balance is maintained. The total number of these Cation exchange sites is known as the Cation Exchange Capacity (CEC). The higher this figure the more nutrients a substrate can potentially hold and supply to the solution. Arguable the most important property of a soil, the Cation exchange Capacity can be expressed in terms of its total exchange and effective exchange capacities as well as its Percentage Base Saturation: this being the relative percentage of the clay’s exchange sites being occupied by the elements and compounds Calcium, Magnesium, Potassium, Sodium and Ammonia. Ideally for optimum availability the PBS should contain 60-80% Ca, 10-15% Mg, 1-5% K and less than 15 % Na (Kasica 1997) (Lippert 2002).

 

Free Cultural Works (CC-BY-NC-SA) Malcolm McEwen 2011

 

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