Ammonium fixation in interlayer positions in vermiculite reduces plant available N from ammonium fertilizers or from organic matter mineralization. Vermiculite that has been transformed from weathered biotite in recently excavated decomposed granite (DG) materials has a high unsaturated fixation potential due to lack of native ammonium content. Ammonium fixation reduces N available for revegetation projects following construction disturbance. Several locations on various granitic batholiths (granites, granodiorites, quartz diorites, and diorites) were sampled throughout California in areas where revegetation was performing poorly. Sorption rates reached their maxima from 16 to 24 hours after amendment and the average absorption capacity was 52 % of that loaded (approximately 49 mmol/kg). In contrast to studies on agricultural soils, the saprolitic DGs appeared to have larger fixation capacities than the soils, and much longer residence time of added ammonium in interlayer positions.