Simple Diffusion Methods for Determination of Nonexchangeable Ammonium in Soils

Core Ideas Existing HF methods are labor-intensive and do not recover recently fixed NH₄ ⁺-N. Diffusion techniques were developed that are simpler and more quantitative. These techniques permit ¹⁵N analyses of fixed NH₄ ⁺-N in tracer studies. The determination of fixed ammonium (NH₄ ⁺) normally involves hydrofluoric acid (HF) digestion to decompose clay minerals following a rigorous pretreatment to control interference by organic nitrogen (N); however, the methods are tedious and time-consuming, and recoveries can be incomplete if the soil under analysis was recently fertilized with ammoniacal N. Diffusion methods were developed to simplify and improve this determination, which utilize a potassium chloride (KCl) pretreatment to clearly differentiate exchangeable from nonexchangeable NH₄ ⁺. In these methods, the HF digest is treated with magnesium carbonate (MgCO₃) to neutralize residual acidity and precipitate magnesium fluoride (MgF₂), and fixed NH₄ ⁺-N is then liberated with magnesium oxide (MgO) as diffusion is performed on an orbital shaker for 3 h at 45 to 50°C (Method A) or for 3 d at 20 to 25°C (Method B). When averaged across 16 diverse soils, both diffusion methods were in close agreement with the Silva-Bremner technique using a potassium hypobromite (KOBr) pretreatment, although significant discrepancies were more common for Method A than B, indicating greater potential for organic interferences. In a study to compare recoveries of recently fixed ¹⁵NH₄ ⁺-N, Method A was significantly higher than the procedure of Silva and Bremner. The latter finding leaves no doubt about the need to avoid the use of KOBr for measuring fixed NH₄ ⁺ in soils recently fertilized with ammoniacal N, and demonstrates that the methods described will be advantageous for tracer research involving ¹⁵N.