Soil Cation Exchange & pH Buffering Simulator

Soil texture

Amend the soil

Dose per click 2.0 meq/100g

CEC

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Base saturation

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Soil pH

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Buffer capacity β

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How it works

Clay and humus particles carry a fixed number of negatively charged exchange sites — the soil's cation exchange capacity (CEC), measured in meq/100g. Those sites hold exchangeable cations: base cations (Ca²⁺, Mg²⁺, K²⁺, shown blue) or acid cations (H⁺, Al³⁺, shown red). Adding acid displaces base cations off the sites and replaces them with H⁺; adding lime does the reverse. The exchange complex behaves like a weak-acid buffer, so pH follows the Henderson–Hasselbalch relation applied to the fraction of sites occupied by base cations.

f = baseMeq / CEC          (base saturation)
pH = pKa + log10( f / (1−f) )
β  = 2.303 · CEC · f · (1−f)   (buffer capacity, meq/100g per pH unit)

Soil texture — sets CEC. Sand has almost no exchange sites; clay and peat have many, so the same dose of acid or lime moves their pH far less.
Add acid / Add lime — moves meq of cations between the base and acid pools, exactly as fertilizer or lime application would in the field.
Buffer capacity β — how many meq/100g it takes to shift pH by one unit right now; it peaks near base saturation ≈ 50%, which is why partially-depleted soils resist further pH change the most.

Real-world relevance: agronomists use exactly this CEC/base-saturation logic to calculate lime requirement — how much lime a field needs to raise pH to a crop's target, scaled by that soil's own buffering capacity.

Base cation (Ca²⁺/Mg²⁺/K⁺) Acid cation (H⁺/Al³⁺) Clay/humus micelle