Estimating Plant-Available Potassium in Potassium-Fixing Soils

Soil tests commonly used to develop potassium (K) fertilizer recommendations, such as extraction by 1 M ammonium acetate (NH₄OAc) at pH 7, measure both soluble and exchangeable K. In soils that fix K so that it is no longer exchangeable, although a portion of it may still be available to plants, additional tests are required to understand the dynamics of soil K. The sodium tetraphenylboron method has been shown in some studies to provide a better measure of plant-available, nonexchangeable K. Two K-fixation-potential tests estimate the amount of K that a soil will fix in a nonexchangeable state. This study uses extractions with ammonium acetate and sodium tetraphenylboron along with the 1-h K-fixation-potential test to analyze the fate of K applied as potassium chloride (KCl) to K-fixing soil samples from the San Joaquin Valley of California. Samples were incubated moist for 1, 2, 4, 8, or 16 days to evaluate the effect of time on the K-test values. The effects of drying on K-fixation potential and NH₄OAc-extractable K were also investigated. After application of K at a rate equal to the measured K-fixation potential and incubation for up to 16 days, soils continued to fix added K, though reduced as compared to initial amounts. Drying treated samples increased their subsequent K-fixation potential but had mixed effects on NH₄OAc-extractable K. Over the range of incubation times, K-fixation-potential values remained stable for all soils. NH₄OAc-extractable K and sodium tetraphenylboron–extractable K values, however, were affected variably by incubation time. The results of this work will be useful for determining K-fertilizer application requirements for K-fixing soils.