Plant-available potassium assessment with a modified sodium tetraphenylboron method

The conventional 1 M NH(4)OAc-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 NH(4)OAc method with a modified NaBPh(4) 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 NH(4)OAc method and the modified NaBPh(4) 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 NaBPh(4)-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 NH(4)OAc-extractable K(+) only in soils with low nonexchangeable K(+) contribution (r(2) = 0.889 and 0.915, respectively), but they were well related to NaBPh(4)-extractable K(+) in all soils (r(2) = 0.984 and 0.874, respectively). Slopes for NH(4)OAc-extractable K(+) vs. soil K(+) balance varied widely among soils (0.16-0.68) depending on NH(4)OAc-extractable K(+), illitic K(+), and clay content, but for NaBPh(4)-extractable K(+) slopes were near unity. These studies suggest that the modified NaBPh(4) method may be a superior K(+) soil test compared to the NH(4)OAc method. Illite content and CEC data may help in developing better soil K(+) management guidelines.