Soil Cation Exchange Capacity Lab

Soil Composition

Clay content 25%

Organic matter (humus) 3.0%

Soil pH 6.5

Live Readouts

CEC

0 cmol⁺/kg

Base saturation

Dominant cation

—

Leached this session

0 sites

Exchange sites

Ca²⁺ (held tightest) Mg²⁺ K⁺ Na⁺ (held weakest) H⁺ / Al³⁺ (acidity)

How it works

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.

CEC (cmol⁺/kg) ≈ 0.6 × Clay% + 2.5 × OM%
Base saturation = (Ca²⁺+Mg²⁺+K⁺+Na⁺ sites) / CEC × 100%

Each simulated marble 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.

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 leach downward 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.

Rain event: weakly-held cations leaching…

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