Determination of exchangeable potassium in soil using ion-selective electrodes in soil suspensions

Methods of determining exchangeable K+ of soil by mixing extracting solutions and analysing the soil suspension with ion-selective electrodes were developed and evaluated on 30 samples of soils. From preliminary comparisons of the K+ extracted by BaCl2 and NH4OAc solutions and by batch and leaching treatments of soils, we established that suspensions of 5 g soil in 100 ml 0.5 M BaCl2 and single batch treatments of 1 h should be used. The exchangeable K+ was determined with a K-selective, valinomycin-based PVC membrane electrode and electrochemical cells that did or did not include a liquid junction (the reference electrode being a double-junction reference electrode assembly with a 10 M LiOAc salt bridge solution or a Cl-selective electrode, respectively). The Ba-exchangeable K+ values were sensibly the same whether a liquid junction was involved or not, a result that can be attributed to the beneficial effects of the salt bridge and the ionic strength of the extractant. Comparisons of these Ba-exchangeable results with those obtained by various combinations of batch or leaching treatments, BaCl2 or NH4OAc extractants and filtrate analysis by the ion-selective electrode method or atomic absorption spectrometry showed they were all highly correlated (r greater than or equal to0.996). The selectivity of the K+-selective electrode (k(KNH4)(pot)=0.004) significantly reduced the interference from indigenous soil NH4+ in the BaCl2 suspensions. Overall, the results show potentiometric measurements of K+ in soil suspensions can provide a simple, rapid, and reliable means of determining exchangeable K+ in soils.