Cation exchange capacity (CEC)
Biochar increases the cation exchange capacity of soil
Biochar surfaces carry oxygenated functional groups with negative charge. Added to soil, the amendment raises the soil's overall CEC, which in turn reduces leaching of cations like potassium, calcium, magnesium, and ammonium.
What the literature says
Biochar surfaces develop oxygenated functional groups over time (confirmed spectroscopically in Cheng 2006), giving them a negative surface charge. Liang et al. (2006) quantified the contribution of black-carbon fractions to total soil CEC in Amazonian terra preta soils and showed black carbon can account for a substantial share of the cation exchange in aged biochar-rich soils.
Mechanism
Negative surface sites on biochar bind positively charged ions — potassium (K⁺), calcium (Ca²⁺), magnesium (Mg²⁺), ammonium (NH₄⁺). Those ions remain available to plant roots but do not flush through the soil profile as easily during heavy rain.
What this means for a gardener
- On sandy soils: substantially reduced leaching of dissolved nutrients between rain or irrigation events.
- On compost-heavy gardens: the ammonium portion of compost-derived nitrogen is held longer before it converts and cycles out.
- On container mixes: fertiliser applied to a biochar-amended container leaches less quickly than an unamended mix.
Important caveats
Fresh biochar has limited CEC; the functional-group density builds over months as surfaces oxidise. The Cheng 2006 work is the reason charging methods (compost blend, compost tea) accelerate this process — the moist, microbially active conditions in charging speed oxidation of biochar surfaces.