Plant-available potassium assessment with a modified sodium tetraphenylboron method

The conventional 1 M NH4OAc-exchangeable potassium (K+) soil test is inadeq uate in soils where nonexchangeable K+ contributes significantly to crop nu trition. Studies were conducted (i) to compare the abilities of the I M NH4 OAc method with a modified NaBPh4 method to estimate critical soil K+ level s, (ii) to estimate the contribution of nonexchangeable K+ to plant-availab le Iii, (iii) to compare the abilities of the 1 M NH4OAc method and the mod ified 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+ ba lance. Winter wheat (Triticum aestivum L. 'Abe') was grown in eleven Midwes tern 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 Kkg(-1)). Ammonium acetate- and NaBPh4-extractable K+ (5-min extraction peri od) were determined in soil samples taken after every three defoliation cyc les and after incubation, Critical soil K+ levels could not be determined b y either method alone but could be predicted by including cation-exchange c apacity (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 KI on ly in soils with low nonexchangeable K+ contribution (r(2) = 0.889 and 0.91 5, respectively), but they were well related to NaBPh4-extractable KI in al l soils (r(2) = 0.984 and 0.874, respectively). Slopes for NH4OAc-extractab le K+ vs, soil K+ balance varied widely among soils (0.16-0.68) depending o n NH4OAc-extractable K+, illitic K+. and clay content, but for NaBPh4-extra ctable K+ slopes were near unity, These studies suggest that the modified N aBPh4 method may be a superior Kt soil test compared to the NH4OAc method. Illite content and CEC data may help in developing better soil Kt managemen t guidelines.