Clay and humus particles carry a permanent negative surface charge (broken bonds and dissociated –COOH / –OH groups). Positively charged nutrient ions are electrostatically attracted to that surface and held in a swappable "exchange" layer — this is Cation Exchange Capacity (CEC), the soil's nutrient-storage battery. This 2D view is a vertical cross-section through a soil sample: clay platelets and humus chunks are scattered through the profile, each studded with colored exchange sites.
CEC (cmol⁺/kg) ≈ 0.6 × Clay% + 2.5 × OM%
Base saturation = (Ca²⁺+Mg²⁺+K⁺+Na⁺ sites) / CEC × 100%
Each dot is one exchange site. Sites are filled with base cations (Ca, Mg, K, Na) or, as pH drops, replaced by acidic H⁺/Al³⁺ — soils below pH 5.5 lose base saturation because acidic cations out-compete nutrients for the same sites. The number of sites drawn scales across the whole clay/OM range so a low-CEC sample visibly looks sparse and a high-CEC sample visibly looks dense — not just a small wobble in dot count, but a real change you can see at a glance.
Cations aren't held equally: the exchange (lyotropic) selectivity series ranks retention strength roughly Al³⁺ > Ca²⁺ > Mg²⁺ > K⁺ > Na⁺ — higher charge and a smaller hydrated radius bond more tightly. When water percolates through the profile (a rainfall event), weakly-held ions like Na⁺ and K⁺ desorb and sink downward out of the exchange layer far more readily than Ca²⁺, which is why sandy, low-CEC soils lose fertility fast while clay- and humus-rich soils buffer it.
- Clay content — more clay means more permanent-charge surface area and a higher baseline CEC.
- Organic matter — humus carries roughly 4× the CEC per unit mass of clay minerals, so even a small increase raises nutrient-holding capacity sharply.
- Soil pH — lower pH converts base cation sites to H⁺/Al³⁺ acidity, cutting base saturation.
- Rainfall event — probabilistically strips sites by selectivity order and refills them with H⁺, mimicking real nutrient leaching after heavy rain.
Real-world relevance: agronomists measure CEC and base saturation in every soil test to decide lime and fertilizer rates — a low-CEC sandy field needs small, frequent nutrient applications, while a high-CEC clay/humus field can be fertilized less often because it holds nutrients between rains.
Drag the profile to pan, scroll (or pinch) to zoom.