Discussion 55b Determination of total mineralized nitrogen

Some workers use 20 g air-dry soil, add 12 ml water and incubate in the dark at 22°C for 28 days, making good any moisture loss. Others extract the air-dry soil directly. The given method uses the fresh (or thawed) soil sample. The usual extractant for moist soils is 2 M KCl, but for dry soil, 10% m/v KCl (1.34 M) may be used. The amount of nitrite-N in the soil is usually so small that it can be neglected, but it will be incorporated in the amount of nitrate recorded.

There are two approaches to the analysis. In the first, the extractable ammonium-N is determined by a Kjeldahl distillation, and this is subtracted from the value for ammonium plus nitrate (and nitrite) determined by a further separate Kjeldahl distillation preceded by reduction with nascent hydrogen produced by the reaction of Devarda's alloy (45% Al, 50% Cu, 5% Zn) in strongly alkaline solution:

production of hydrogen: 2Al + 2OH- + 2H2O = 2AlO2- + 3H2T

overall reaction: 3NO3- + 8Al + 5OH- + 2H2O = 8AlO2- + 3NH3

The second, quicker, way is simply to add the Devarda's alloy to the distillation flask after the first distillation, and then redistil the ammonia produced from the reduced nitrate - this is described below.

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