Cation Exchange Capacity

CEC is essentially the soil's nutrient-holding capacity. Cations adhere electrostatically to negatively charged clay particles and humus surfaces; as plant roots absorb one cation (say K⁺), they release H⁺ that can displace another held cation back into the soil solution. Soils with high CEC can stockpile large amounts of nutrients without leaching them into groundwater, while low-CEC sandy soils lose fertilizer quickly after rain.

Typical ranges: sand 2–4 meq/100g, sandy loam 5–10, loam 10–15, clay loam 15–25, clay 25–40, organic/peat 50–100. A standard soil test reports CEC alongside base saturation — the percentage of the CEC occupied by Ca, Mg, K, and Na (versus acidity from H⁺ and Al³⁺). Optimal base saturation is roughly 65–80% Ca, 10–15% Mg, 3–7% K, <3% Na.

CEC determines fertilizer strategy. On high-CEC soils, a single large fertilizer application can last a full season. On low-CEC sandy soils, split applications (3–5 smaller doses through the growing season) prevent leaching losses. Building soil organic matter is the primary way to raise CEC over time — each 1% SOM increase adds roughly 2–4 meq/100g to total CEC.