Nonexchangeable ammonium and potassium determination in soils with a modified sodium tetraphenylboron method

The mechanism of NH4+ and K+ fixation by clay minerals is similar, yet different analytical methods are used to estimate nonexchangeable (fixed) amounts of these nutrients in soils, Sodium tetraphenylboron (NaBPh(4)) facilitates the release of nonexchangeable NH4+ and K+ by precipitation of these ions as NH(4)BPh(4) and KBPh(4). A NaBPh(4) method was modified and evaluated as a common procedure for determination of nonexchangeable NH4+ and K+ in 10 midwestern soils that were either untreated or NH4+- or K+-saturated (treated with 2 M solutions of NH4Cl or KCl to saturate fixation sites with NH4+ or K+). Experiments were conducted to evaluate Cu2+ (CuCl2) as a substitute for Hg2+ (HgCl2) in the recovery of NH4+ and K+ precipitated by NaBPh(4) as NH(4)BPh(4) and KBPh(4). The results showed that 5 mmol CuCl2 can be used instead of HgCl2 to recover NH4+ and K+ released by NaBPh(4) in soils. A 7-d incubation with the modified NaBPh(4) method (CuCl2 instead of HgCl2) released most of the NaBPh(4)- extractable NH4+ and K+. In untreated soils, the NaBPh(4) method extracted 71% of NH4+ released by the conventional HF/HCl method. Nonexchangeable NH4+ release by both methods was related in untreated (r(2) = 0.98) and NH4+-saturated (r(2) = 0.95) soils. Recently fixed NH4+ in the NH4+-saturated soil was equally accessible to both methods. The NaBPh(4) method extracted up to 10 times more nonexchangeable K+ than the conventional 1 M HNO3 method in both the untreated and K+-saturated soils. Recently fixed K+ in the K+-saturated soil was equally accessible to both methods, except in high-fixing soils where K+ was held too strongly to be released by HNO3. The results show that the modified NaBPh(4) method can be used as a common procedure to estimate nonexchangeable forms of both NH4+ and K+.