Cation Exchange Resins for Predicting Available K on K-Deficient Soils: Extraction Capacity among Different Soil K Pools and First Insights on the Contribution of K Solubilized by Rhizosphere Microbes

The contribution of non-exchangeable soil K in the total K availability may be essential to K deficient soils; however, less attention has been paid so far for proposing soil tests that assess the bioavailable structural K that is solubilized by soil microbes during a growing season. The objectives of this study were to evaluate cation exchange resins for their performance in predicting K uptake by plants; to investigate the relationships between rhizosphere soil microbes, different soil K pools, and soil parameters; and to assess their exchange capacity with respect to K derived by feldspars. A pot experiment with winter wheat was conducted on K deficient soils, several soil tests were examined, and rhizosphere K solubilizing microbial population was assessed. Single and multiple regressions showed that cation resins performed better in predicting total plant K uptake than the other chemical extractants ( r 2 = 0.64, r 2 = 0.85, respectively, p ≤ 0.001), whereas the PCA analysis and Pearson correlation tests revealed a positive correlation between K derived by feldspars, K uptake, and the K solubilizing rhizosphere microbial population. The above was further confirmed by the mapping of cation resins of extraction capacity, which showed a significant contribution of K derived by feldspars (15.6%).