Differences of Phosphorus in Mehlich 3 Extracts Determined by Colorimetric and Spectroscopic Methods

Abstract

Mehlich 3 (M3) is a widely used extractant for evaluating plant available phosphorus (P) in soils and may be quantified using colorimetric or inductively coupled plasma (ICP) spectroscopic methods. Analysis by ICP has recently become increasingly popular in soil‐testing labs primarily due to its ability to simultaneously measure multiple elements. Despite the versatility and efficiency of ICP, some laboratories hesitate to use ICP to determine P in M3 extracts because (1) fertilizer recommendations were developed using colorimetric analysis, and (2) differences have been reported between P analyzed colorimetrically and by ICP. This study documented the differences between M3 P measured colorimetrically using a flow injection auto‐analyzer and by ICP (ICP‐P) for approximately 6400 routine soil samples analyzed by the Soil, Water and Forage Analytical Laboratory at Oklahoma State University. Potential factors contributing to the difference in colorimetric P and ICP‐P were also examined using another set of 100 well‐characterized soils. Mean ICP‐P was greater than colorimetric P (p<0.001) for both sets of soils. Highly significant relationships existed between colorimetric P and ICP‐P for the 6400 soils (r2=0.98, p<0.001) and for the 100 soils (r2=0.97, p<0.001). The relative difference between colorimetric P and ICP‐P decreased exponentially with increasing (soil test P measured) colorimetrically. Significant relationships did not exist (p>0.05) between the relative difference in colorimetric and ICP‐P and soil properties, including soil pH, clay content, organic carbon content, or M3 extractable aluminum (Al), calcium (Ca), and iron (Fe). Fertilizer recommendations based on colorimetric P predicted by ICP values were highly related (r2=0.81, p<0.001) to fertilizer recommendations based on actual colorimetric P.