Do we properly quantify P from Mehlich 1 or Mehlich 3 extracting solutions?

Abstract

Abstract Aims The performance of colorimetric methods for P quantification is usually overlooked by researchers and not suitable to errors. Here, we investigated the performance of two blue colorimetric methods for P quantification, Braga & Defelipo (1974) (B&D) and Murphy & Riley (1962) (M&R), using two commonly soil P extractants, Mehlich-1 (M1) and Mehlich-3 (M3). Methods Specifically, we evaluated the color development time and its stability in different soil extractant solution proportions (1:1 or 1:4 v/v), the optimum wavelength, limits of detection and element quantification. Results Our results indicate that M3 leads to lower limits of detection and quantification for both colorimetric methods, particularly for B&D. For M1, the volumetric ratio (1:1 or 1:4) did not influence color development and both B&D and M&R methods showed a fast color development. However, B&D showed greater color stability (from 5 to 600 min) and an optimum wavelength of 711 nm, while M&R was stable from 27 to 600 min an optimum wavelength of 889 nm. For M3 soil extractant, there are important issues, such as M&R being unstable and B&D presenting slow color development. In addition, the spectral profile obtained from soil extractant was different from the one obtained by the calibration curve without soil for both M&R and B&D. Conclusions Therefore, the adoption original methods on P quantification in M3 extracting solution represents a potential source of error, leading to wrong P fertilizer recommendations. Thus, the best option for P quantification in M3 extracts seems to be the use of ICP.