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Core Ideas Original ascorbic acid methods are sensitive in STP solutions at phosphate <2 mg L−1. New optimum wavelengths for the Dick and Tabatabai method in STP extraction solutions. A new optimum wavelength of 888 nm for Asher's methods is recommended. Phosphate spectrophotometric methods are valuable in agro‐environmental research because of the orthophosphate‐specific detection. While optimum wavelength and detection limits for these methods are suggested in ultrapure water, they have rarely been evaluated in soil test P (STP) extracting solutions such as Bray P‐1 and Mehlich III. When four different phosphate spectrophotometric methods (Murphy and Riley, He et al., Dick and Tabatabai, and Asher) were evaluated in these STP extracting solutions, we found (i) that the methods of Murphy and Riley and He et al. are still sensitive at P concentrations <2 mg L−1 in STP solutions, (ii) a new optimum wavelength (886–888 nm) in STP solutions for the Dick and Tabatabai method, and (iii) a new optimum wavelength and limit of detection for Asher's methods in ultrapure water and STP solutions. The new optimum wavelengths and detection limit reported in this study should be useful for colorimetric‐based orthophosphate analysis.

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