Determination of Nitrate and Exchangeable Ammonium in Soil Extracts by an Ammonia Electrode1
- R. S. Siegel2
A method for determination of NO3- in waters using an NH3 electrode was adapted for use with soils. Nitrate was determined in 1 N KCl soil extracts as the difference in NH3-N concentration between aliquots treated with NaOH and Devarda's alloy to reduce NO3- and aliquots made alkaline without addition of Devarda's alloy. The rate of reduction of NO3- to NH3 was temperature dependent. The minimum recommended temperature for the procedure is 23°C. Rate of NH3 loss from solution and NH3 activity measured by the NH3 electrode increased with temperature.
Nitrate-N in extracts of nine Illinois soils determined with the NH3 electrode was highly correlated (r2 = 0.999***)3 with NO3--N analyzed by the phenoldisulfonic acid method. Exchangeable NH4+-N determined with the modified electrode filling solution was highly correlated (r2 = 0.99***) with exchangeable NH4+-N determined by steam distillation. In addition to NO3-, NO2- is reduced to NH3 by this procedure. Recovery of added NH4+, NO3-, and NO2- was 97, 97, and 91%, respectively. Precision of NO3- and exchangeable NH4+ determinations were 0.2 and 0.1 mg/kg, respectively. The reagents used are relatively inexpensive and safe to handle, and reduction is carried out at room temperature. Only 20 ml of soil extract is required and soils containing 1 to 250 mg NO3--N/Kg can be handled routinely. One worker can analyze 80 extracts/day.
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