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DIVISION S-8-NUTRIENT MANAGEMENT & SOIL & PLANT ANALYSIS

Quantitative Determination of Recently Fixed Nonexchangeable Ammonium in Soils

^a Univ. of Florida, Institute of Food and Agricultural Sciences, 700 Experiment Station Rd., Lake Alfred, FL 33850, present address, Center for Marine, Environmental Sci., and Biotechnology Research, Savannah State Univ., P.O. Box 20288, Savannah, GA 31404 USA
^b Dep. of Agronomy, Madison Sturgis Hall, Louisiana State Univ., Baton Rouge, LA 70803 USA

siva{at}savstate.edu

The importance of nonexchangeable NH₄ as a potential source of plant-available N is widely recognized. However, quantitative determination of nonexchangeable NH₄ in soils poses a challenging analytical problem. The use of the KOBr pretreatment in the methods currently used to determine nonexchangeable 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 samples reduced recovery of fixed NH₄ 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 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 amounts of fixed NH₄ 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 NH₄, 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 NH₄ during KOBr oxidation. In the absence of KOBr pretreatment, determination of NH₄ by steam distillation resulted in values inflated by the partial recovery of organic N. The proposed method 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.

Abbreviations: RCF, relative centrifugal force • RF, recently fixed

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				G. D. Eudoxie and G. A. Gouveia Microdiffusion of Strongly Fixed Ammonium in Soil-Acid Digests Prior to Automated Nitrogen-15 Analysis Soil Sci. Soc. Am. J., November 1, 2001; 65(6): 1846 - 1852. [Abstract] [Full Text] [PDF]