Cation exchange capacity is a measure of a potting mix's ability to retain positively charged ions, i.e. cations. Cations such as K⁺, Ca²⁺, Mg²⁺, NH⁴⁺, Mn²⁺, Fe²⁺ and Cu²⁺ are all key elements needed for healthy plant growth. These cations are loosely held on the negatively charged surface of colloids comprising the humus (fine organic matter), or clay (soil) components of the mix. Plant roots release H⁺ cations which exchange with the other cations held on the colloid surface close to the roots. These freed cations are then taken up by the roots. Mixes with a high cation exchange capacity will also have the greatest capacity to resist or buffer the acidifying effect of fertilizers.

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