‘A’ Values of Potassium Related to Other Indexes of Soil Potassium Availability

A technique was developed to measure ‘A’ values of K with oat ( Avena sativa ) plants grown in the greenhouse and with the radioactive isotope 42 K. The isotope 42 K has a short half‐life (12.42 ± .03 hours) and was applied to the soil as KCl (approximately 2 mc of 42 K/g of 39 K) 14 to 18 days after seeding. Plants were harvested to determine initial quantities of K at the time of tagged fertilizer application. Six days later the fertilized plants were harvested and the activity in the plant tissue was counted in a liquid scintillation spectrometer. For 11 soils the total of exchangeable K plus nonexchangeable K accounted for 56% of the variability in K uptake at the time of tracer application, whereas 92% of the variability in ‘A’ values was accounted for by these measurements. Results from the experiments indicated that both amounts of exchangeable and nonexchangable K should be used as availability indexes.