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

Dep. of Agronomy, Purdue Univ., West Lafayette, IN 47907-1150

*Corresponding author (bjoern{at}dept.agry.purdue.edu).

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⁺.

Journal Paper no. 14592 of the Indian Agricultural Research Program, West Lafayette.

Received for publication March 13, 1995.

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