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Determination of Nitrate and Exchangeable Ammonium in Soil Extracts by an Ammonia Electrode¹

ABSTRACT

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.

¹ Contribution from the Agronomy Department and Ill. Agric. Exp. Stn. Univ. of Ill., Urbana, IL 61801.

² Associate Agronomist, Department of Agronomy, Univ. of Ill. Urbana, IL 61801.

³ *** Indicates significance at the 0.001 level.

Received for publication September 21, 1979. Accepted for publication April 18, 1980.

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