It is the sum of exchangeable cations , or positively charged ions, media can adsorb per unit weight or volume. A high CEC value characterizes media with a high nutrient-holding capacity that can retain nutrients for plant uptake between applications of fertilizer. Media characterized by a high CEC retains nutrients from leaching during irrigation. The cations bind loosely to negatively charged sites on media particles until they are released into the liquid phase of the media. Once they are released into the media solution, cations are absorbed by plant roots or exchanged for other cations held on the media particles.
Some media retains small quantities of anions , negatively charged ions, in addition to cations.