Plant-Available Potassium Assessment with a Modified Sodium Tetraphenylboron Method

The conventional 1 M NH₄OAc-exchangeable potassium (K⁺) soil test is inadequate in soils where nonexchangeable K⁺ contributes significantly to crop nutrition. Studies were conducted (i) to compare the abilities of the 1 M NH₄OAc method with a modified NaBPh₄ method to estimate critical soil K⁺ levels, (ii) to estimate the contribution of nonexchangeable K⁺ to plant-available K⁺, (iii) to compare the abilities of the 1 M NH₄OAc method and the modified NaBPh₄ method to estimate plant dry matter yield and plant-available K⁺, and (iv) to compare the abilities of both methods to measure soil K⁺ balance. Winter wheat (Triticum aestivum L. `Abe') was grown in eleven Midwestern soils in a greenhouse using consecutive 28-d defoliation and regrowth cycles. Soils also were incubated for 6 mo with five K⁺ rates (0–809 mg K⁺ kg⁻¹). Ammonium acetate- and NaBPh₄-extractable K⁺ (5-min extraction period) were determined in soil samples taken after every three defoliation cycles and after incubation. Critical soil K⁺ levels could not be determined by either method alone but could be predicted by including cation-exchange capacity (CEC) and illitic K⁺ content in regression models. Nonexchangeable K⁺ represented a significant portion of plant-available K⁺ Plant-available K⁺ and dry matter (DM) yield were well related to NH₄OAc-extractable K⁺ only in soils with low nonexchangeable K contribution (r ² = 0.889 and 0.915, respectively), but they were well related to NaBPh₄-extractable K⁺ in all soils (r ² = 0.984 and 0.874, respectively). Slopes for NH₄OAc-extractable K⁺ vs. soil K⁺ balance varied widely among soils (0.16–0.68) depending on NH₄OAc-extractable K⁺, illitic K⁺, and clay content, but for NaBPh₄-extractable K⁺ slopes were near unity. These studies suggest that the modified NaBPh₄ method may be a superior K⁺ soil test compared to the NH₄OAc method. Illite content and CEC data may help in developing better soil K⁺ management guidelines.