The aim of this investigation is to study how adsorbed humus on clay minerals affects kinetics of dissolution of clay mineral by an organic acid in terms of K and Si release. Clay–humus complexes were isolated from soil samples collected from different depths of 2 soil profiles from an Inceptisol of North India under long-term rice–wheat and maize–wheat cropping systems. Clay–humus complexes were subjected to repeat extraction with 100 mg/L of tartaric acid solutions before and after removal of humus with 30% hydrogen peroxide and the resultant release of potassium and silica was measured.

Before removal of humus, K release followed Elovich kinetics for the maize–wheat cropping system.For the rice–wheat cropping system, except surface soil, which followed Elovich kinetics, other subsurface soils followed the power form of kinetic equations. But when humus was partially removed, K release increased exponentially, therelease rate increasing with time. This indicated that for humus-depleted clay, initial release triggered further release of K.

Silica was released at a constant rate. The dissolution reaction was incongruent, i.e. the amount of constituents released in solution was not proportional to their mole fraction present in the mineral. X-ray diffraction analysis showed that micaceous mineral was partially dissolved and vermiculite was completely dissolved when humus was partially removed from clay before extraction. This observation indicates that humus adsorbed on clay plays an important role in preventing clay dissolution by organic acids.