Accelerated diffusion methods for inorganic-nitrogen analysis of soil extracts and water

The major limitation of diffusion methods for N analysis is the lime required to obtain the results - typically from one to several days. Methods were developed that involve a much shorter period fur diffusion. In these methods, 10- to 100-mL samples of 2 or 4 M KCl soil extract or water were treated with MgO, or MgO plus Devarda's alloy, in a 473-mL (1-pint) wide-mouth Mason jar to liberate NH₃-N₄ NO₃-N₄ and/or NO₂-N as NH₃. The jar was placed on an electric griddle maintained at 45 to 50°C. Gaseous NH₃ was collected in 5 mL of H₃BO₃ indicator solution in a petri dish attached to the Mason jar lid, for quantitative determination by acidimetric titration. Complete recovery of 4 mg of N was achieved from 10 or 20 mL in 1.5 to 5 h. Recovery from 50 or 100 mL was complete in 5 to 8.5 h with 1 to 2 mg of N. The methods permit ¹³N analysis of the diffused NH₃-N. Isotopic analyses of labeled soil extracts were accurate to within 3%, as determined by isotope-dilution calculations. Diffusions from 10 to 50 mL were subject to very little, if any, interference from decomposition of alkali-labile organic-N compounds.