Characterization of soil potassium with ion-selective electrodes

Wang, Jian
Major Professor
A. Duncan Scott
Committee Member
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This study was initiated to (1) develop a potentiometric procedure of using ion-selective electrodes (ISEs) to determine soil exchangeable potassium (K) in soil suspensions, (2) compare and improve the ISE methods of determining soil potassium Q/I relationships, and (3) characterize the effects of experimental procedures and other factors on the potentiometrically determined Q/I relationships for soil K;It was established that batch treatments of 5-g samples of Iowa soils in 100 ml 0.5 M BaCl[subscript]2 solutions for 1 hr and subsequent determinations of the extracted K in the BaCl[subscript]2-soil suspensions with K-ISEs in electrochemical cells that did or did not include liquid junctions can yield exchangeable K[superscript]+ results that are highly correlated with (r = 0.996) and not significantly different from ([alpha] = 0.05) those obtained with an NH[subscript]4OAc leaching procedure and a leachate analysis by atomic absorption spectrometry (AAS);Quantity-intensity (Q/I) relationships for K in Iowa soils were determined with a successive CaCl[subscript]2-KNO[subscript]3 or -KCl additions procedure, a 10-min equilibration period after each K addition and ISE methods of analyzing the CaCl[subscript]2-soil suspensions. The results obtained with single-ISE (ISE(S)), dual-ISE (ISE(D)) and triple-ISE (ISE(T)) methods were very much alike and similar to those determined by AAS analysis of soil filtrates. Nevertheless, it was concluded that the ISE(T) method provides the best means of carrying out simple, rapid and reliable potentiometric determinations of potassium Q/I relationships with soil suspensions;Major changes in the Q/I relationships for K in soils were encountered when the equilibration period was varied. These changes were identified with an increase in K[superscript]+ sorption when the systems with high cation concentration ratios (CR) were equilibrated longer and a reversion of K[superscript]+ when air-dry soil samples were rewet with solutions that had low CR values. In response to these changes in exchangeable K[superscript]+, the equilibration period had much more effect on the Q/I parameters of soil samples deeper in the profile. Even so, it was concluded that Q/I measurements should be carried out with a successive K addition procedure, a 10-min equilibration period after each K addition and a soil suspension analysis with the ISE(T) method.