Evaluation of Manual Cadmium Reduction Methods for Determination of Nitrate in Potassium Chloride Extracts of Soils

Abstract

AbstractAlthough 2M KCl is widely used as an extractant for inorganic N in soils, most colorimetric NO‐3 methods are not applicable to such extracts. Keeney and Nelson (1982) have recently described a Cd reduction procedure for determination of NO‐3 in KCl extracts that was adapted from those used for water analysis. Other published manual Cd reduction methods may be applicable to analysis of KCl extracts of soils. The objectives of this work were to determine the conditions necessary for quantitative reduction of NO‐3 on the Cd column and to evaluate the applicability of Cd reduction procedures for the determination of NO‐3 in KCl extracts. Studies indicated that slower eluant flow rates and/or longer Cd columns than those specified in the Keeney and Nelson method resulted in nonquantitative recovery of added NO‐2 and NO‐3 due to destruction of NO‐2 by the Cd column. These findings indicated that the Keeney and Nelson method was the manual Cd reduction procedure best suited for analysis of KCl extracts. Nitrate added to 2M KCl extracts of soils was quantitatively recovered by the Keeney and Nelson method. The coefficient of variation for the method ranged from 2.1 to 3.4% when KCl extracts were analyzed over 3 d. Analysis of NO‐3 in 2M KCl extracts of soils established that the Cd reduction method gave values similar to those obtained by a steam distillation‐acidometric titration procedure and markedly higher that those with a brucine colorimetric technique. After KCl extracts had been prepared, approximately 36 samples could be analyzed per hour by the Keeney and Nelson procedure. The manual Cd reduction method using a 110 mL min−1 flow rate and a 20‐cm column is a rapid, precise, and accurate technique for determination of NO‐3 in KCl extracts of soils.