Direct‐Diffusion Methods for Inorganic‐Nitrogen Analysis of Soil

Abstract

Determinations of exchangeable NH4, NO3, and NO2 in soil normally require the preparation of an extract, which is usually obtained with a neutral salt solution such as 2 M KCl. In cases where analytical speed and convenience are key factors, as in routine soil testing, a more practical approach would be to carry out the determination directly on the soil sample itself. Studies were conducted to ascertain whether this approach could be employed using Mason‐jar diffusion methods previously developed for inorganic‐N analysis of soil extracts. Direct diffusions were performed with MgO to recover NH4–N, or with MgO and Devarda's alloy to recover (NH4 + NO3 + NO2)–N, from 1‐ to 5‐g samples of a wide variety of air‐dried soils, following treatment of the soil with sufficient 2 M KCl to give a 1:10 ratio of soil to solution. Up to 4 mg of inorganic N was recovered quantitatively in 18 h to 6 d at room temperature, or in 1.75 to 5.5 h with gentle heating on a hot plate (45–50°C). Analyses by direct diffusion were in close agreement with results obtained by diffusion of soil extracts, and recoveries ranged from 97 to 102% when direct diffusions were performed on samples that had been treated with NH4–N or NO3–N. Besides being quantitative, direct diffusions were found to have potential for N‐isotope analysis. In diffusing 15N‐treated samples, isotope‐ratio analyses of NH4–N or (NH4 + NO3 + NO2)–N were accurate to within 3%. The latter finding virtually precludes interference by labile organic N or nonexchangeable NH4–N; however, interference did lead to an increase in inorganic‐N analyses when diffusion was prolonged beyond the recommended period.