Effect of experimental relevance on potassium Q/I relationships and its implications for surface and subsurface soils

Abstract

Recent methodological developments for determining potassium quantity–intensity (Q/I) relationships employing ion-selective electrodes offer a rapid means of carrying out such measurements for routine soil testing. In this investigation, various experimental factors in rapid measurements and their relevance to surface and subsurface soils are considered. Increasing the equilibration period generally enhanced the degree to which K+ was released or adsorbed by surface soil samples. Most of the gain or loss of K (ΔK) in the soil solution occurred within the first 10 min of the equilibration period. Also, regression analysis for 32 Iowa soils demonstrated that the Q/I results were highly correlated between short (e.g., 10 min) and long (e.g., 18 hr) equilibration periods. The correlations were even better when the results for surface and subsurface soils were considered separately. Increasing soil sampling depth generally increased the potential buffering capacity but had no consistent effect on other Q/I parameters. The depth of the soil samples also modified the effects of equilibration periods on the Q/I results. For subsurface soils, the 18-hr equilibration period usually induced less K+ release in low concentration ratio (CR) values in contrast to what occurs in surface soils. Such a distinction in Q/I status was attributed to the difference in the degree of reversion of released K+ between surface and subsurface soils during air-drying and re-wetting of samples. Overall, the results suggest that soil potassium Q/I relationships can be assessed with short equilibration periods but the interpretations for fertility status of surface and subsurface soils should be evaluated separately.