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

Recent methodological developments for determining potassium quantity-inten sity (Q/I) relationships employing ion-selective electrodes offer a rapid m eans of carrying out such measurements for routine soil testing. In this in vestigation, various experimental factors in rapid measurements and their r elevance to surface and subsurface soils are considered. Increasing the equ ilibration period generally enhanced the degree to which K+ was released or adsorbed by surface soil samples. Most of the gain or loss of K (AK) in th e soil solution occurred within the first 10 min of the equilibration perio d. Also, regression analysis for 32 Iowa soils demonstrated that the Q/I re sults were highly correlated between short (e.g., 10 min) and long (e.g., 1 8 hr) equilibration periods. The correlations were even better when the res ults for surface and subsurface soils were considered separately. Increasin g 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 resu lts. For subsurface soils, the 18-hr equilibration period usually induced l ess K+ release in low concentration ratio (CR) values in contrast to what o ccurs 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 an d subsurface soils during air-drying and re-wetting of samples. Overall., t he results suggest that soil potassium Q/I relationships can be assessed wi th short equilibration periods but the interpretations for fertility status of surface and subsurface soils should be evaluated separately.