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Absence of Local Equilibrium During Ammonium Transport in a Soil Column¹

ABSTRACT

Adsorption of NH₄⁺ from a stationary solution phase by a Hanford soil approached an equilibrium state exponentially after about 30 min. By contrast, a local equilibrium did not exist between NH₄⁺ in a moving solution phase and soil in a column while a 75-ppm NH₄⁺-N solution was delivered at a constant flow rate for 26 days. Local equilibrium, however, was established when the downward solution flow was stopped by setting the column horizontally 1 hour before sampling. Thus, the generality of the assumption of instantaneous equilibrium for cations in moving soil solutions is questioned.

¹ Contribution from the College of Natural Resources, Univ. of California, Berkeley. Supported by the Kearney Foundation of Soil Science.

² Assistant Research Chemist and Professor of Soil Biology, respectively, Dep. of Soils and Plant Nutrition, Univ. of California, Berkeley, CA 94720. The current address of the first author is Center for the Biology of Natural Systems, Washington Univ., St. Louis, MO 63130.

Received for publication September 13, 1976. Accepted for publication April 22, 1977.