Abstract
Clay‐fixed NH4–N can represent up to 18 percent of the total soil N for Canadian agricultural soils. A modification of the Silva and Bremner's method for determining clay‐fixed NH4 was developed. The Silva and Bremner procedure incorporates dissolution of mineral residues by a HF–HCl digestion in order to release bound NH4, following removal of exchangeable NH4 and organic N. Steam distillation is used to recover NH4 from the soil digest. The procedure presented in this paper substitutes combustion in an automated elemental analyzer for the HF–HCl digestion step. Combustion with an elemental analyzer follows removal of exchangeable NH4 with 0.5 M KCl and organic N with wet digestion using KOBr. The elimination of the HF–HCl digestion and steam distillation steps shortens the time required for analysis and avoids the use of strong reagents. The procedure is also suitable for 15N studies when combined with a mass spectrometer. The two techniques gave virtually the same results for 17 soils with the slope of the regression of 1.03 and a coefficient of determination of 0.97. Simultaneous measurements of fixed NH4 and N isotope ratios using continuous flow mass spectrometry can be carried out.
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