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Determination and Isotope-Ratio Analysis of Different Forms of Nitrogen in Soils: 5. Fixed Ammonium¹

ABSTRACT

A 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.

¹ Journal Paper No. J-5367 of the Iowa Agr. and Home Econ. Exp. Sta., Ames. Projects 1070 and 1450. This work was supported in part by the Tennessee Valley Authority.

² Formerly Graduate Assistant and Professor, respectively, Department of Agronomy, Iowa State University, Ames. The senior author is now Assistant Professor of Soils, University of Hawaii, Honolulu.

Received for publication April 4, 1966. Accepted for publication May 20, 1966.

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