What is Sorption in soils?

The cation exchange capacity of a soil is the amount of exchangeable cations it can hold per unit mass, on clays and organic matter. The soil–water distribution coefficient $K_d$ is the ratio of the concentration of a substance sorbed on the soil (per kilogram) to its concentration in the pore water (per litre); the organic-carbon partition coefficient is $K_{oc} = K_d/f_{oc}$, with $f_{oc}$ the mass fraction of organic carbon.