Quantitative determination of recently fixed nonexchangeable ammonium in soils

The importance of nonexchangeable NH4 as a potential source of plant-availa ble N is widely recognized. However, quantitative determination of nonexcha ngeable NH4 in soils poses a challenging analytical problem. The use of the KOBr pretreatment in the methods currently used to determine nonexchangeab le NH4 in soils was found to cause a substantial loss of this fraction. For example, a second KOBr pretreatment prior to digestion of air-dried sample s reduced recovery of fired NH4 by 6.4 to 32.8%, Therefore, a more reliable method was developed that avoids KOBr pretreatment. The proposed method qu antitatively recovers (98-99%) nonexchangeable NH4 including recently fixed (RF) as well as native clay fixed NH4 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 amount s of fixed NH4 were determined in 17 air-dried soil samples that ranged wid ely in clay and organic matter content, However, when these samples were tr eated to enrich RF NH4, values obtained by methods employing KOBr were an a verage of 35% less than those obtained by the proposed method. These differ ences were attributed to loss of RF NH4 during KOBr oxidation, In the absen ce of KOBr pretreatment, determination of NH4 by steam distillation resulte d in values inflated by the partial recovery of organic N, The proposed met hod determines the amount of NH4-N in 5 M HF:1 M HCl soil digests by a NH3 membrane diffusion technique that is highly accurate, minimizes recovery of organic N, and avoids the health hazards posed by the use of KOBr.