The soil's capacity to hold positively charged nutrients like potassium, calcium and magnesium — higher CEC buffers better; low (sandy) CEC leaches faster.
Cation exchange capacity measures how many positively charged nutrients (K⁺, Ca²⁺, Mg²⁺, NH₄⁺) your soil's clay and organic matter can hold. A high-CEC clay or muck soil holds a big reservoir and resists change; a low-CEC sandy soil holds little, leaches faster, and benefits from split applications.
CEC also shapes lime and potassium behavior, which is why some state methods adjust recommendations by CEC or texture. It's context that makes a soil test meaningful — the same ppm means different things at different CEC.
Related: Mehlich-3 · Buildup & maintain