Comparison of Ion-Selective Electrode Methods for Determining Potassium Q/I Relationships

Methods of characterizing the K + status of soils in terms of quantity-intensity (Q/I) relationships usually entail lengthy equilibration periods, filtrations, and multiple analyses of the filtrates. It has been established, however, that simpler and less time-consuming methods of determining Q/I parameters can be devised with ion-selective electrodes (ISEs). To this end, different ISE methods were developed and evaluated with suspensions of three Iowa soils and a successive-additions method of varying the added K+. The ISE methods, which were categorized on the basis of the electrochemical cell involved, were tested for their applicability to soil suspensions. Specifically, single-ISE (K- and Ca-ISEs vs. a double-junction reference electrode assembly with a 10 M LiOAc salt bridge), dual-ISE (K-ISE vs. Ca-ISE), and triple-ISE (K- and Ca-ISEs vs. Cl-ISE) methods were compared to ascertain the best means of determining the ΔK (gain or loss of dissolved K) and CR [concentration ratio: CK/(CCa+Mg)1/2] terms of the Q/I relationships. The Q/I curves for the three soils and the specific parameters of Q/I relationships were also determined with the different ISE methods. It was concluded that the combined approach of using the triple-ISE method with soil suspensions offers the best means of carrying out simple, rapid, and liquid junction free determinations of potassium Q/I relationships.