Thje way that I understand it is that the Cation Exchange Capacity is a measure of the soils ability to hold and supply nutrients. The organic matter and clay fractions of the soil have chemical structures that produce sites with partial negative electrical charges. These are cation exchange sites. Positively charge ions, such as potassium, magnesium, calcium, hydrogen, ammonium, and sodium are called cation. The cations are attracted to the sites negative charges and held there. The cation exchange capacity is the amount of cations a soil can hold and is expressed as milliequivalents per 100 grams of soil. Basically it shows how much nutrients each soil particle can hold onto and absorb. A higher CEC is better. With a low CEC then you will have to fertilize less and more often. This is at least what I have found out through my research.