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Miscible Displacement of Ammonia in Soil; Determining Sorption Isotherms¹

ABSTRACT

Although anhydrous ammonia is used extensively as a common source of nitrogen for crop production, there is probably less known concerning its behavior after application to soils than that of any other nitrogenous fertilizer. Most techniques used in the past for evaluating the sorption characteristics of anhydrous ammonia have utilized steady-state or so-called equilibrium conditions without considering a method that allows or accounts for simultaneously-occurring mass flow, diffusion, and sorption of the ammonia gas. A miscible displacement technique was developed that entails the displacement of the atmosphere from oven-dry soil columns with ammonia at flow velocities of approximately 100 cm hr^-1.

Transient sorption isotherms calculated from miscible displacement experiments were compared with the measured steady-state and transient sorption isotherms for both adsorption and desorption. Isotherms relating the amount of sorption S to the relative ammonia density P (or concentration) were of the nonlinear from S = kPⁿ + k_o where k, k_o, and n were sorption constants different for adsorption and desorption.

¹ Contribution from the Department of Water Science and Engineering, University of California, Davis.

² Assistant Professor of Soil Physics, Dep. of Soils & Plant Nutrition, and Professors of Water Science, Dep. of Water Science & Engineering, University of California, Davis 95616.

Received for publication April 6, 1971. Accepted for publication June 22, 1971.