EXTRACTABLE RUBIDIUM IN SURFACE HORIZONS OF ILLINOIS SOILS

Several studies suggest that extractable Rb at very low levels may play an important role in K activity. Rubidium extracted with normal neutral NH4OAc was determined in a suite of surface horizons to characterize its concentration relative to soil-forming factors and to extractable K. Mean extractable Rb (n = 59) was 138 mumol kg-1 soil, and accounted, on the average, for 1.31% of total Rb. Alfisols had significantly less extractable Rb than Mollisols, which was attributed to lower clay contents and a higher degree of weathering in the Alfisols. The mean atom percent of extractable Rb relative to extractable Rb plus K was 0.58% (range 0.07-1.40). Available K, as measured by the fertility quick-test procedure, was inversely proportional to the atom percent of extractable Rb, perhaps because a portion of the extractable Rb had occupied nonextractable sites in phyllocilicates with the addition of K fertilizer.