Simple Diffusion Methods for Determination of Nonexchangeable Ammonium in Soils

Core Ideas Existing HF methods are labor‐intensive and do not recover recently fixed NH 4 + ‐N. Diffusion techniques were developed that are simpler and more quantitative. These techniques permit 15 N analyses of fixed NH 4 + ‐N in tracer studies. The determination of fixed ammonium (NH 4 + ) 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 NH 4 + . In these methods, the HF digest is treated with magnesium carbonate (MgCO 3 ) to neutralize residual acidity and precipitate magnesium fluoride (MgF 2 ), and fixed NH 4 + ‐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 15 NH 4 + ‐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 NH 4 + in soils recently fertilized with ammoniacal N, and demonstrates that the methods described will be advantageous for tracer research involving 15 N.