Nonexchangeable Ammonium and Potassium Determination in Soils with a Modified Sodium Tetraphenylboron Method
The mechanism of NH+4 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 (NaBPh4) facilitates the release of nonexchangeable NH+4 and K+ by precipitation of these ions as NH4BPh4 and KBPh4. A NaBPh4 method was modified and evaluated as a common procedure for determination of nonexchangeable NH+4 and K+ in 10 midwestern soils that were either untreated or NH+4 − or K+-saturated (treated with 2 M solutions of NH4Cl or KCl to saturate fixation sites with NH+4 or K+). Experiments were conducted to evaluate Cu2+ (CuCl2) as a substitute for Hg2+ (HgCl2) in the recovery of NH+4 and K+ released by NaBPh4 in soils. A 7-d incubation with the modified NaBPh4 method (CuCl2 instead of HgCl2) released most of the NaBPh4- extractable NH+4 and K+. In untreated soils, the NaBPh4 method extracted 71% of NH+4 released by the conventional HF/HCl method. Nonexchangeable NH+4 release by both methods was related in untreated (r2 = 0.98) and NH+4-saturated (r2 = 0.95) soils. Recently fixed NH+4 in the NH+4-saturated soil was equally accessible to both methods. The NaBPh4 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 NaBPh4 method can be used as a common procedure to estimate nonexchangeable forms of both NH+4 and K+.
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