Determination of Nitrate and Exchangeable Ammonium in Soil Extracts by an Ammonia Electrode

Abstract A method for determination of NO 3 ‐ in waters using an NH 3 electrode was adapted for use with soils. Nitrate was determined in 1 N KCl soil extracts as the difference in NH 3 ‐N concentration between aliquots treated with NaOH and Devarda's alloy to reduce NO 3 ‐ and aliquots made alkaline without addition of Devarda's alloy. The rate of reduction of NO 3 ‐ to NH 3 was temperature dependent. The minimum recommended temperature for the procedure is 23°C. Rate of NH 3 loss from solution and NH 3 activity measured by the NH 3 electrode increased with temperature. Nitrate‐N in extracts of nine Illinois soils determined with the NH 3 electrode was highly correlated ( r 2 = 0.999***) with NO 3 ‐ ‐N analyzed by the phenoldisulfonic acid method. Exchangeable NH 4 + ‐N determined with the modified electrode filling solution was highly correlated ( r 2 = 0.99***) with exchangeable NH 4 + ‐N determined by steam distillation. In addition to NO 3 ‐ , NO 2 ‐ is reduced to NH 3 by this procedure. Recovery of added NH 4 + , NO 3 ‐ , and NO 2 ‐ was 97, 97, and 91%, respectively. Precision of NO 3 ‐ and exchangeable NH 4 + 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 NO 3 ‐ ‐N/Kg can be handled routinely. One worker can analyze 80 extracts/day.