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Published in Soil Sci Soc Am J 51:664-667 (1987)
© 1987 Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
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The Effect of Calcium and Magnesium on Soil Nitrate Determination by Automated Segmented-flow Methods1

S. Ananth and J. T. Moraghan2

ABSTRACT

An automated procedure, involving the reduction of NO-3 to NO-2 by an alkaline solution of hydrazine sulfate containing traces of Cu2+, is commonly used for determination of small quantities of NO-3-N in electro-ultrafiltration (EUF) extracts of soils. When this reduction method was employed to determine NO-3-N in aqueous extracts from certain Calciaquolls, low and erratic results were obtained with 24 of 46 soil samples. The problem extracts contained high levels of Ca2+ and Mg2+ that interfered with the method. Soil extracts with <40 mg Ca2+ L–1 and 15 mg Mg2+ L–1 presented no analytical problems. Placement of a cation exchange resin column between the sampler and pump of a Technicon AutoAnalyzer II overcame the interference, due presumably to removal of Ca2+ and Mg2+ from the flow line. Solution studies revealed that Ca2+ and especially Mg2+ prevented quantitative reduction of NO-3 to NO-2. No analytical problems were encountered with any of the soil extracts when a Cd-Cu column was used to reduce NO-3 to NO-2. Any hydrazine sulfate reduction method should be carefully evaluated when newly introduced as an analytical procedure for soil NO-3-N determination.

NOTES

1 Contribution from Dep. of Soil Science, North Dakota State Univ., Fargo, ND 58105. Published with the approval of the Director as Journal Series no. 1503.

2 Graduate Research Assistant and Professor, respectively.

Received for publication July 10, 1986.




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