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Abstract Colorimetric analysis of nitrate in soil extracts using chromotropic acid (CTA) was studied and compared to the results obtained by the phenoldisulfonic acid (PDA) method. Extracting solutions of 0.5 M MgSO 4 , saturated CaSO 4 , and a 0.2% Ca(OH) 2 suspension were investigated. The three extractants were equally effective in recovering NO 3 ‐ ; however, the clearest extracts were obtained with Ca(OH) 2 . Soil‐to‐extractant ratios of 1:2.5, 1:5, and 1:10 were studied. The 1:5 ratio effectively extracted NO 3 ‐ and resulted in appropriate NO 3 ‐ concentrations for color development. A shaking time of 15 min was adequate to extract a maximum of NO 3 ‐ . In a NO 3 ‐ ‐N recovery study, equivalent amounts of NO 3 ‐ were recovered by the CTA and PDA methods. Results of nitrate analyses by the two methods were significantly correlated ( p = .01). Nitrate determined by the chromotropic acid procedure was positively correlated ( p = .05) with the yield of winter wheat ( Triticum aestivum L.) and was negatively correlated ( p = .01) with response of winter wheat to N fertilizer additions. The new procedure has the advantages of rapidity, simplicity, sensitivity, and relative freedom from interference.
Sims et al. (Thu,) studied this question.