Cation exchange capacity, or CEC, is your first chemistry term of the day. The CEC is the soil's ability to hold onto positively charged chemicals called cations. It's the chemical current that will deliver the needed goods to plants that will, in turn, make them flourish. Some of those important chemicals include calcium, magnesium, and sodium. In layman's terms, the CEC has a direct correlation to the overall fertility of the soil. The lower the CEC, the lower the fertility.