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Mineral Fixation of Anhydrous NH₃ by Air-Dry Soils¹

ABSTRACT

Fixation of anhydrous NH₃ by the mineral fraction of air-dry samples from 65 horizons in 17 Pacific Northwest soils was investigated. Soils represented a wide variety of great soil groups. Nitrogen fixed from anhydrous NH₃ was compared to (a) nitrogen fixed from aqua-NH₃, (b) indigenous fixed NH₄⁺, and (c) NH₃ retention of the whole soil (organic and mineral fraction).

Mineral fixation of anhydrous NH₃ varied greatly between soils and between horizons within individual profiles. Values ranged from 6 to 1015 ppm. N and appeared large enough to have possible practical significance in some soils. Though small in comparison to NH₃ retention, mineral fixation of NH₃ caused lattice collapse from 14Å. to 10Å. in some samples. Mineral fixation of anhydrous NH₃ by air-dry samples generally exceeded by severalfold the wet fixation of nitrogen from aqua ammonia.

Ammonia retention ranged from 820 to 9190 ppm. N. Of the NH₃ retained, from 1 to 8% in the surface and from 2 to 31% in the subsurface horizons was fixed by the mineral fraction.

Indigenous fixed NH₄⁺, found in all horizons, ranged from 17 to 125 ppm. N and comprised from 1 to 7.5% and from 2 to 42% of the total N in surface and subsurface horizons, respectively. From 11 to 89% and from 6 to 63% of the total mineral-fixed NH₃ in the surface and subsurface horizons, respectively, was previously satisfied by indigenous fixed NH₄⁺.

¹ Joint contribution of the Soil and Water Conservation Research Division, ARS, USDA, and the Oregon Agr. Exp. Sta., Corvallis. Technical Paper No. 1370, Oregon Agr. Exp. Sta. The work was partially supported by Phillips Petroleum Co., Bartlesville, Okla.

² Chemist, Western Soil and Water Management Research Branch, SWCRD, ARS, USDA, and Research Assistant, Oregon Agr. Exp. Sta., Corvallis, respectively. Present address of Junior author: Department of Soils and Agricultural Chemistry, University of Arizona, Tucson.

Received for publication March 20, 1961. Accepted for publication September 2, 1961.