Potential uses of ion-selective potassium electrodes in soil fertility management

Mapping horizontal spatial variability in soil-potassium (K) fertility is driving renewed interest in measuring soil K with ion selective K electrodes (ISE-K). The objectives of this study were 1) to develop a rapid and inexpensive ISE-K protocol for soil solution phase [K+], 2) to compare this protocol to published methods for assessment of soil K intensity and capacity, and 3) to explore electrode sensitivity to environmental variables including temperature and soil moisture content. Using 32 agricultural soils with a wide range in plant-available K, we evaluated two combination electrodes, a glass electrode (ISE-K-Gls) similar to that used in earlier soil applications and a PVC membrane flat-surface electrode (ISE-K-PVC) that is being considered for on-the-go real-time sensing of soil K. A 15-s agitation 30-min settling proved sufficient to equilibrate K concentrations in a 1:1 H2O soil/water w/v extraction. The ISE-K readings in slurries, supernatants, and filtrates were equivalent and did not differ from atomic adsorption spectrophotometer (AAS) filtrate analyses. Among all soils, ISE-K accurately predicted solution-phase [K+] differences as identified by three common reference methods used with AAS, but individual ISE-K measured values were higher than any AAS-measured value. To prevent interference by Na+ and NH4+, NaCl had been added as an ionic strength adjuster (ISA) and the added Na+ displaced K+ off exchange surfaces, elevating soil solution [K+]. On naturally leached soils, the ISA may be less important as concentrations of Na+ and NH4(+) tend to be low. Results from ISE-K measurements in soils equilibrated at 20% gravimetric moisture without ISA support this contention. These ISE-K values closely tracked AAS-measured soil solution [K+] (regression coefficient=0.93; r(2)=0.76). Among soils, ISE-K measured values were not well correlated with NH4OAc or NaBPh4 extractable K, an expected result given differences in soil mineralogy and associated buffering capacity. In general, electrode output varied predictably with changes in ambient temperatures and gravimetric soil moisture indicating appropriate corrections in [K+] measurements could be made if these parameters were simultaneously monitored. The ISE-K-Gls retained calibration more consistently than the ISE-K-PVC, and the ISE-K-PVC proved less durable, especially for measurements made in soil slurries. The rapid extraction of soil solution K coupled with ISE-K technology is sufficiently precise and accurate for both research and routine use. However, general application of this technology either in the commercial testing lab or on-the-go in the field will require that soil fertility management guides be revised to include interpretive information for soil solution-phase [K+].