Quantitative Determination of Recently Fixed Nonexchangeable Ammonium in Soils

The importance of nonexchangeable NH 4 as a potential source of plant‐available N is widely recognized. However, quantitative determination of nonexchangeable NH 4 in soils poses a challenging analytical problem. The use of the KOBr pretreatment in the methods currently used to determine nonexchangeable NH 4 in soils was found to cause a substantial loss of this fraction. For example, a second KOBr pretreatment prior to digestion of air‐dried samples reduced recovery of fixed NH 4 by 6.4 to 32.8%. Therefore, a more reliable method was developed that avoids KOBr pretreatment. The proposed method quantitatively recovers (98–99%) nonexchangeable NH 4 including recently fixed (RF) as well as native clay fixed NH 4 while avoiding significant recovery (<1%) of N in soil organic matter or in microbial cells. Values obtained by the proposed method were similar to methods employing KOBr when the amounts of fixed NH 4 were determined in 17 air‐dried soil samples that ranged widely in clay and organic matter content. However, when these samples were treated to enrich RF NH 4 , values obtained by methods employing KOBr were an average of 35% less than those obtained by the proposed method. These differences were attributed to loss of RF NH 4 during KOBr oxidation. In the absence of KOBr pretreatment, determination of NH 4 by steam distillation resulted in values inflated by the partial recovery of organic N. The proposed method determines the amount of NH 4 –N in 5 M HF:1 M HCl soil digests by a NH 3 membrane diffusion technique that is highly accurate, minimizes recovery of organic N, and avoids the health hazards posed by the use of KOBr.