Use of Indigenous Soil‐Rubidium Absorbed by Cotton Plants in Determining Labile Soil‐Potassium Pool Sizes

The rubidium/potassium (Rb/K) ratios of natural soils and of most soil‐grown plants are higher than Rb/K ratios of many available fertilizer‐grade potassium salts as revealed by analyses of 24 samples of commercial K‐fertilizers. Low‐Rb K‐fertilizers of 0.02 mmoles Rb/mole K or lower may offer practicable means for field scale tracing of fertilizer‐K absorbed by crop plants when growing under normal management regimes. Whenever K‐fertilization results in lower Rb/K ratios appearing in fertilized plants, this dilution or “reversed‐tagging” of indigenous soil‐Rb with low Rb‐containing K‐fertilizers permits determinations of pool‐sizes of labile soil‐K. Four different soils were compared for NH 4 OAc extractable‐K, HNO 3 extractable‐K, and the pool‐size of labile‐K as determined from Rb/K ratios of K‐fertilized cotton plants ( Gossupium hirsutum L.). A large pool size of labile‐K does not necessarily mean that the K supplying power of a soil is large. The Rb/K ratios can also be used to calculate relative amounts of absorbed plant‐K originating from the indigenous soil‐pool vs. the plant‐K derived from a K‐fertilizer.