A Sensitive Method for the Measurement of Ammonium in Soil Extract and Water Hojeong Kang, 1, * Emily H. Stanley, 1 and Seok-Soon Park 2 1 Center for Limnology, University of Wisconsin, Madison, Wisconsin, USA 2 Department of Environmental Science and Engineering, Ewha Institute of Science and Technology, Ewha Womans University, Seoul, Korea ABSTRACT Ammonium is one of the most important inorganic nutrients in many ecosystems. The utility of a sensitive method for the measurement of ammonium (NH þ 4 ) in water, soil and plant-derived samples using fluorescence of an o-phthaldialdehyde (OPA)-based compound was evaluated in this study. This method involves less toxic chemicals than a conventional method, requires minimal equipment, and allows for fairly rapid sample processing. This method is not only sensitive for low (e.g., sub-micromolar) NH þ 4 concentrations, but also applicable for samples with much higher concentrations (. 100 mM NH þ 4 ) by using a modified spectrophotometric protocol developed herein. This evaluation demons- trates that this method provides a valuable tool for the measurement of 2193 DOI: 10.1081/CSS-120024057 0010-3624 (Print); 1532-2416 (Online) Copyright q 2003 by Marcel Dekker, Inc. www.dekker.com * Correspondence: Hojeong Kang, Department of Environmental Science and Engineering, Ewha Institute of Science and Technology, Ewha Womans University, Seoul 120-750, Korea; Fax: 82-2-3277-3275; E-mail: hjkang@mm.ewha.ac.kr. COMMUNICATIONS IN SOIL SCIENCE AND PLANT ANALYSIS Vol. 34, Nos. 15 & 16, pp. 2193–2201, 2003 MARCEL DEKKER, INC. • 270 MADISON AVENUE • NEW YORK, NY 10016 ©2003 Marcel Dekker, Inc. All rights reserved. This material may not be used or reproduced in any form without the express written permission of Marcel Dekker, Inc.