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Titratable Acidity to Estimate Ammonia Retention¹

ABSTRACT

Simulation models of fertilizer behavior in soils are potentially useful tools to improve fertilizer use efficiency. Presently, there are no satisfactory models that describe NH₃ retention in soils. In order to develop such a model, a better understanding of how soils retain NH₃ and a quantitative measure of soils' NH₃ retention capacity are needed. We compared soil cation exchange capacity (CEC) and several measures of soil titratable acidity for predicting the amount of NH₃ retained in the center of a band resulting from anhydrous NH₃ application to soil. The titratable acidity to pH 9 estimated with NH₄OH was a much better predictor than CEC of the NH₃ retained. In some soils, the pH in the center of an NH₃ retention zone differed slightly from pH 9, resulting in some error in estimating the NH₃ retained. This error was greatest on a salty soil because of the small slope of the buffer curve near pH 9. We found soil titratable acidity to be stoichiometrically related on a 1:1 basis to the amount of NH₃ retained. Apparently, CEC is a poor predictor of the amount of NH₃ retained by soil because many of the cation exchange sites do not participate in a soil's capacity to adsorb NH₃. Since a soil's titratable acidity to pH 9 is a stoichiometric predictor of NH₃ retention capacity in soils, it may be used to more easily model the properties of an NH₃ retention zone as affected by factors such as N fertilizer rate applied, spacing between application points, and soil bulk density.

¹ Contribution no. 87-1-J, from the Kansas Agric. Exp. Stn. Part of a dissertation submitted by the senior author in partial fulfillment of the requirements for the Ph.D. degree at Kansas State Univ. This study was supported by a grant from the Test and Demonstration Branch of the National Fertilizer Development Center of TVA.

² Former Graduate Research Assistant, Professor, and Graduate Research Assistant, Dep. of Agronomy, Kansas State Univ., Manhattan, KS 66506, respectively. Address of the senior author is Dep. of Soil Science, Univ. of Alberta, Edmonton, Alberta, Canada T6G 2E3.

Received for publication July 28, 1986.