AMMONIUM FIXATION BY SOIL AND PURE CLAY-MINERALS

Fixation of the ammonium ion (NH4+) by clay minerals is an alternate way of building the nitrogen (N) pool in soil to optimize N crop recovery and minimize losses. Clay minerals (illite, montmorillonite, and vermiculite) and an illitic Portnoeuf soil were used to compare NH4+ fixation abilities. Total N determination and X-ray diffraction analysis were performed on each of the minerals and the Portnoeuf soil controls, and NH4+ saturated batches were subsequently desorbed by potassium chloride (KCl) after 4096 hours. Total N was determined for each employing either Kjeldahl digestion only, or pretreating with hydrofluoric-hydrochloric acid (HF-HCl) before the Kjeldahl digestion. The total N for the soil was 38% more after pretreatment with HF-HCl. The total N determined after pretreatment with HF-HCl for the NH4+ saturated and subsequently KCl desorbed minerals was found to be highest in vermiculite. The cation exchange acapacity (CEC) of each of the minerals was determined, and highest CEC was found in montmorillonite [83.07 cmol(+)/kg]. X-ray diffraction analysis revealed collapse of the vermiculitic clay lattice from an initial d-spacing of 13.1 angstrom to 10.4 angstrom after desorption by KCl. This suggested the existence of sequestered NH4+ between the 2:1 vermiculitic clay interlayer lattice.