HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS		QUICK SEARCH:   [advanced] Author: Keyword(s): Year:  Vol:  Page:

This Article Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Farrell, R. E. Articles by Scott, A. D. Search for Related Content PubMed Articles by Farrell, R. E. Articles by Scott, A. D. Agricola Articles by Farrell, R. E. Articles by Scott, A. D.

Ion-selective Electrode Determinations of Exchangeable Potassium in Soils¹

ABSTRACT

Procedures using ion-selective electrodes (ISEs) for the accurate determination of NH⁺₄ -and Ba²⁺-exchangeable K⁺ in soil extracts are described. The exchangeable K⁺ in 30 soil samples was extracted with neutral solutions of 1.0 M NH₄OAc and 0.5 M BaCl₂ and determined by atomic absorption spectrometry (AAS) to establish the K⁺ levels sought with the electrodes. Potentiometric measurements were then made with electrochemical cells consisting of either a valinomycin-based K⁺-selective electrode (VKE) or a cationic glass-membrane electrode (CGE) and a double-junction sleeve-type reference electrode assembly with a saturated LiOAc salt bridge. The electrochemically analyzed solutions (1:5 extract/water dilutions) had a common ionic strength and a pH near 7 or a LiOH-adjusted pH of 11.6. A comparison of the AAS values for the Ba²⁺- and NH⁺₄-exchangeable K⁺ demonstrated the feasibility of using either extractant for an evaluation of ISE methods. Interferences from NH⁺₄ and Li⁺ limited the use of the CGE to the neutral BaCl₂ extracts, where it yielded exchangeable K⁺ values that were only poorly correlated (r = 0.655) with those obtained by AAS. However, the Ba²⁺-exchangeable K⁺ values obtained with the VKE were highly correlated (r = 0.983) with and not significantly different ( = 0.05) from those obtained by AAS with the neutral extracts and even more so with solutions at pH 11.6 (r = 0.998; = 0.01). Accurate direct-potentiometric determinations of the exchangeable K⁺ in the NH₄OAc extracts were not feasible, even at pH 11.6. But the NH⁺₄-exchangeable K⁺ values obtained with the VKE and a modified standard additions technique were highly correlated (r = 0.987) with and not significantly different ( = 0.05) from those obtained by AAS.

¹ Journal Paper no. J-12432 of the Iowa Agric. and Home Econ. Exp. Stn., Ames, IA. Project 2692. Presented in part before Div. S-2, Soil Sci. Soc. Am., Washington, DC., 15 Aug. 1983. This work was supported in part by the Potash and Phosphate Inst.

² Graduate Research Assistant and Professor, respectively. Dep. of Agronomy, Iowa State Univ., Ames, IA 50011. The senior author is currently a Postdoctoral Research Associate at Iowa State Univ.

Received for publication September 29, 1986.