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Use of Indigenous Soil-Rubidium Absorbed by Cotton Plants in Determining Labile Soil-Potassium Pool Sizes¹

ABSTRACT

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 NH₄OAc extractable-K, HNO₃ 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.

¹ Contribution from the Dep. of Soils and Plant Nutrition and Kearney Foundation of Soil Science, Univ. of California, Davis 95616. The work reported here is a part of the Ph.D. thesis by the senior author.

² Former Research Assistant, now Postdoctoral Research Soil Scientist, and Professor of Soil Sci., Univ. of California, Davis, respectively.

Received for publication July 11, 1972. Accepted for publication April 10, 1973.

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