Retention and fixation of NH3 and NH4+ by 76 horizon samples from 20 different Pacific Northwest soils were investigated using aqua and anhydrous NH3, NH4Cl, and different degassing and fixing conditions.
Retention of NH3 by ammoniated air-dry samples was approximately 0.7 me. N per each me. of soil cation-exchange capacity. In the surface horizons, retention of NH3 was roughly 0.3 and 2.6 me. N per 100g. of soil for each percent clay and organic carbon, respectively.
Ratios of “NH3 retained by air-dry ammoniated samples” to “H2O retained by the air-dry samples prior to ammoniation” ranged from 0.03 to 0.19 but were generally < 0.1.
As with expandable lattice minerals, vacuum degassing increased NH3 retention substantially beyond that of free diffusion degassed samples open to moisture of the atmosphere. Loss of retained NH3 by diffusion and/or water vapor displacement continued at a slow rate for many weeks and while total retention was decreasing, mineral-fixed values were generally increasing.
Infrared absorption patterns were completely dominated by the soil mineral fraction thereby nullifying possible determination of sites or functional groups reactive in NH3 fixation. An absorption band peaking near 7µ appeared most closely correlated with mineral-fixed NH4+.
Generally, three-fourths or more of the retained anhydrous NH3 was KCl leachable. Of the fixed NH3 (that not leachable with KCl) the major part was associated with the organic fraction except in most subsurface horizons of the recent silty alluvial and water-deposited silt soils wherein the major part was fixed by the mineral fraction.
Comparative NH3/NH44 fixation revealed widely variable behavior, suggesting that a considerable gradation of mineral structures may be involved in soil NH3/NH4+ reactions and that a correspondingly broad range in availability of fixed NH3/NH4+ is to be expected. A number of the minerals giving significant fixation exhibited characteristics intermediate between specimen montmorillonites and vermiculites, implicating beidellitic and/or expanding vermiculite-like structures.