Determination and Isotope‐Ratio Analysis of Different Forms of Nitrogen in Soils: 5. Fixed Ammonium

Abstract

AbstractA method of determining fixed ammonium in soils is described. The soil sample is treated with alkaline KOBr solution to remove exchangeable ammonium and organic nitrogen compounds, and the residue is washed with 0.5M KCl and shaken with 5N HF:1N HCl for 24 hr, the ammonium released by the HF‐HCl treatment being determined by steam distilling the soil‐acid mixture with KOH. The KOBr treatment effects almost quantitative removal of organic soil nitrogen under conditions which preclude fixation by soil minerals of ammonium released from soil organic matter by this treatment, and evaluation tests indicate that the procedure used to estimate fixed ammonium in the KOBr‐pretreatment residue is satisfactory. The method is simple and precise, and it has none of the observed defects of previous methods of estimating fixed ammonium in soils. It gives higher values than the Dhariwal‐Stevenson, Bremner, and Mogilevkina methods, and lower values than the Rodrigues and Schachtschabel methods. A modification in which the HF‐HCl treatment is performed at 100C for 30 min is described.