Ammonia and Ammonium Reactions with Some Pacific Northwest Soils

Retention and fixation of NH 3 and NH 4 + by 76 horizon samples from 20 different Pacific Northwest soils were investigated using aqua and anhydrous NH 3 , NH 4 Cl, and different degassing and fixing conditions. Retention of NH 3 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 NH 3 was roughly 0.3 and 2.6 me. N per 100g. of soil for each percent clay and organic carbon, respectively. Ratios of “NH 3 retained by air‐dry ammoniated samples” to “H 2 O 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 NH 3 retention substantially beyond that of free diffusion degassed samples open to moisture of the atmosphere. Loss of retained NH 3 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 NH 3 fixation. An absorption band peaking near 7µ appeared most closely correlated with mineral‐fixed NH 4 + . Generally, three‐fourths or more of the retained anhydrous NH 3 was KCl leachable. Of the fixed NH 3 (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 NH 3 /NH 4 4 fixation revealed widely variable behavior, suggesting that a considerable gradation of mineral structures may be involved in soil NH 3 /NH 4 + reactions and that a correspondingly broad range in availability of fixed NH 3 /NH 4 + 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.