Direct Colorimetric Measurement of Ammonium in Potassium Chloride Extracts of Soils

Abstract Although 2 M KCL is widely used as an extractant for inorganic N in soils, there is no manual colorimetric methods for direct measurement of ammonium‐N (NH + 4 ‐N) in KCl extracts. The indophenol blue method previously used for measurement of NH + 4 ‐N in aqueous solutions has been modified to permit estimation of NH + 4 ‐N in extracts of soils. The method entails the addition of 1 mL of 6% (wt/v) disodium ethylene‐diamine‐tetraacetic acid (EDTA) solution to an aliquot of a 2 M KCl extract of soil followed by addition of phenol‐nitroprusside and buffered hypochlorite reagents. Maxiumum color development occurs in 60 min at room temperature (23°C), but color development time is reduced to 30 min and molar absorptivity of the chromophore is enhanced (60%) by incubation of the sample at 40°C. Although previous studies have shown that precipitates form when indophenol blue color developing reagents were added to KCl extracts of soils, the addition of EDTA eliminated precipitate formation in samples containing as much as 6 mg of Ca 2+ or 4 mg of Mg 2+ per 25 mL of final analytical volume. The proposed procedure quantitatively measures the NH + 4 ‐N present in KCl extracts of soils and is sensitive to concentrations of NH + 4 ‐N as low as 0.2 µg per 25 mL of final analytical volume. The proposed procedure was the most precise of the three NH + 4 ‐N methods (steam distillation and acidimetric titration, steam distillation into phenol‐nitroprusside and NH + 4 ‐N determination by the indophenol blue colorimetric method, and the proposed procedure) studied. The proposed method provides a sensitive, precise, and accurate colorimetric method for direct measurement of NH + 4 ‐N in KCl extracts of soils.