A new approach for removing iron interference from soil nitrate analysis

Wendy H. Yang, Donald Herman, Daniel Liptzin, Whendee L. Silver

Research output: Contribution to journalArticlepeer-review


The accurate measurement of soil nitrate (NO3-) is critical for determining rates of nitrogen (N) cycling and potential N losses from ecosystems. Iron (Fe) can interfere with the colorimetric NO3- analysis of soil extracts to cause the appearance of anomalously low NO3- concentrations. To resolve the interference, imidazole or NH 4Cl-DTPA has been recommended to replace NH 4Cl-EDTA as the buffer in the analysis. Here we show that phosphate (PO43-) filtration can completely remove Fe interference whereas any of these buffers alone may not. Regardless of which buffer was used, 5.5-55mg FeL -1 ferrous iron (Fe 2+) interfered with NO3- determination in 0.3mg N-NO3-L -1 2M KCl solutions. Phosphate filtration removed all detectable Fe 2+ in 2M KCl anaerobic soil slurry extracts with high Fe 2+ concentrations (25.9±1.7mg Fe 2+L -1). With each of the three buffers tested, the measured NO3- concentrations in the anaerobic soil slurry extracts were significantly higher with PO43- filtration compared to without filtration. After filtration, the measured NO3- concentrations were similar across all three buffers, suggesting that NO3- concentrations were accurately measured in PO43- filtered soil extracts regardless of the buffer used. The Fe:N ratio of Fe interference with NO3- determination depended on Fe concentration, NO3- concentration, buffer, and cadmium column age, so that the amount of Fe interference that could occur can be difficult to predict. We suggest comparing measured NO3- concentrations for unfiltered and PO43- filtered soil extracts to determine the potential for Fe interference in colorimetric NO3- determination as standard additions may not detect all forms of Fe interference.

Original languageEnglish (US)
Pages (from-to)123-128
Number of pages6
JournalSoil Biology and Biochemistry
StatePublished - Mar 2012
Externally publishedYes


  • Colorimetric analysis
  • Griess-Ilosvay
  • Imidazole
  • Iron interference
  • NH Cl-DTPA
  • NH Cl-EDTA
  • Nitrate
  • Nitrite
  • Phosphate filtration
  • Soil extract

ASJC Scopus subject areas

  • Microbiology
  • Soil Science


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