| Abstract: | Abstract In view of the agronomic and economic significance of NH4 fixation in soils, an attempt has been made to relate this to the most reactive mineral constituents of soils ‐ the clay minerals, under the temperature‐moisture regimes normal to tropical upland rice soils. Laboratory fixation study was done with NH4, concentrations similar to those common in soils upon N fertilization, and under alternate wetting and drying at ambient temperatures rather than at 100°C as in many published studies. Results of the investigation show that soil clays with dominant vermiculite and montmorillonite fix the greatest proportion of applied NH4 (94 and 91%), followed by beidellite (72%) and x‐ray amorphous (45–64%) clays. Fixation is negligible (10%) in the clay with mineral suite consisting of hydrous mica, halloysite, and chlorite. Crystallinity of minerals seems to influence NH4 fixation appreciably. |
