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Ammonia and Ammonium Reactions with Some Layer-Silicate Minerals¹

ABSTRACT

Retention of NH₃ and fixation of NH₄⁺ by 16 specimen layer-silicate minerals was investigated using anhydrous and aqua NH₃, NH₄Cl, and different degassing and fixing conditions.

Despite a much greater partial pressure differential, vacuum degassing (to remove excess and physically sorbed NH₃) greatly accentuated the retention and apparent NH₃ fixation as compared with diffusion degassing.

The great bulk of NH₃ initially sorbed by anhydrous NH₃-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 NH₃ per acre-furrow-slice, however, was retained longer than 3 days by the expandable layer-silicates. The major portion (68 to 99+%) of retained NH₃ 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 NH₄⁺ 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 NH₃ as from NH₄Cl. All other minerals fixed roughly equivalent amounts from both sources.

Montmorillonites fixed very small quantities of NH₃ or NH₄⁺ under wet or air-dry conditions but showed a roughly 20-fold increase in NH₄⁺ fixation resulting from drying at 105°C.

Anomalous retention and mechanisms of NH₃ 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.

¹ Contribution of the Soil and Water Conservation Research Division, ARS, USDA, in cooperation with the Oregon Agr. Exp. Sta., Corvallis. Technical Paper No. 1710, Ore. Agr. Exp. Sta. Appreciation is expressed for financial assistance from Phillips Petroleum Co., Bartlesville, Okla., and analytical assistance from W. Wipper and L. F. Lee.

² Chemist and Soil Scientist, Northwest Branch SWC, ARS, USDA, Corvallis, Ore. Junior author now Soil Scientist, U. S. Salinity Laboratory, SWC, ARS, USDA, Riverside, Calif.

Received for publication September 18, 1963. Accepted for publication January 23, 1964.