Nonexchangeable Ammonium and Potassium Determination in Soils with a Modified Sodium Tetraphenylboron Method

1 January 1996

journal article
Published by Wiley in Soil Science Society of America Journal

Vol. 60 (1) , 114-120
https://doi.org/10.2136/sssaj1996.03615995006000010019x

Abstract

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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