Ammonia and Ammonium Reactions with Some Layer‐Silicate Minerals

Retention of NH 3 and fixation of NH 4 + by 16 specimen layer‐silicate minerals was investigated using anhydrous and aqua NH 3 , NH 4 Cl, and different degassing and fixing conditions. Despite a much greater partial pressure differential, vacuum degassing (to remove excess and physically sorbed NH 3 ) greatly accentuated the retention and apparent NH 3 fixation as compared with diffusion degassing. The great bulk of NH 3 initially sorbed by anhydrous NH 3 ‐treated layer‐silicates was rapidly dissipated (few hours to < 3 days) when samples were exposed to the atmosphere. Redisplacement by competitive water vapor evidently was a principal driving force. The equivalent of tons of NH 3 per acre‐furrow‐slice, however, was retained longer than 3 days by the expandable layer‐silicates. The major portion (68 to 99+%) of retained NH 3 was KCl leachable in all except the vermiculitic samples in which > 90% was fixed. Wide variability in basal spacing changes resulted from ammoniation and wetting treatments. Physical changes as reflected by X‐ray diffraction were in general accord with chemical data. Vermiculitic minerals under wet or air‐dry conditions fixed quantities of NH 4 + greatly in excess of amounts fixed by other minerals. Metabentonite 38 and biotite 33 fixed moderate quantities under these conditions. In a comparison of N sources under wet fixing conditions, less than 20% as much N was fixed by vermiculite from aqua NH 3 as from NH 4 Cl. All other minerals fixed roughly equivalent amounts from both sources. Montmorillonites fixed very small quantities of NH 3 or NH 4 + under wet or air‐dry conditions but showed a roughly 20‐fold increase in NH 4 + fixation resulting from drying at 105°C. Anomalous retention and mechanisms of NH 3 retention and fixation are briefly discussed. Results re‐emphasized need for continued restraint to minimize overextrapolation of data from a few type minerals to complex mixtures as generally encountered in soils.