Interlaboratory validation of the Mehlich 3 method as a universal extractant for plant nutrients

The Mehlich 3 (M3) method is widely used to extract plant-available phosphorus from soil over a wide range of pH values. The method is also used by many laboratories to determine multiple plant-available nutrients simultaneously. However, except for P, this method has not been statistically validated within and among laboratories. The objective of this study was to determine the repeatability (within-laboratory performance) and reproducibility (among-laboratories performance) of the M3 method for several different nutrients by using a wide variety of soils. An in-house homogeneity test was conducted for 11 soils. Three replicates of each of the 11 soils were sent to 23 domestic and international laboratories for analyses primarily for K, Ca, Mg, Zn, Mn, Fe, and Cu. Samples were scooped, weighed, or both scooped and weighed for extraction. The various nutrients in the extracts were quantified by the participating laboratories by using inductively coupled plasma-atomic emission spectrometry. Ranges (in parentheses) of the repeatability relative standard deviation (RSDr) values for the scooped samples were K (3.88-6.14%), Ca (2.19-10.6%), Mg (2.27-5.73%), Zn (4.11-42.7%), Mn (3.15-8.53%), Fe (2.32-7.74%), and Cu (3.65-11.2%). For the weighed samples, the ranges (in parentheses) of the RSDr values were K (1.65-4.65%), Ca (1.43-16.9%), Mg (1.37-9.83%), Zn (2.60-33.0%), Mn (1.61-4.90%), Fe (1.56-4.47%), and Cu (2.95-17.9%). Ranges (in parentheses) of the reproducibility relative standard deviation (RSDR) values for the scooped samples were K (7.44-20.0%), Ca (7.10-33.5%), Mg (7.63-26.4%), Zn (11.6-48.7%), Mn (10.5-19.7%), Fe (12.4-22.0%), and Cu (8.86-45.3%). Ranges (in parentheses) of the RSDR values for the weighed samples were K (3.47-12.7%), Ca (7.61-34.6%), Mg (7.53-29.0%), Zn (10.6-49.2%), Mn (6.54-20.7%), Fe (11.0-21.5%), and Cu (9.73-43.0%). The Horwitz ratios (HorRat) were also used to evaluate the repeatability, HorRatr, and the reproducibility, HorRatR. Overall, the M3 method appears to be both repeatable and reproducible across the two categories for K, Mg, Zn, Mn, and Cu, and the vast majority of the HorRat values for both repeatability and reproducibility were within the acceptable range. However, a large number of soils in both the weighed and scooped categories displayed unacceptable HorRat values for the reproducibility of Ca and Fe, indicating that the M3 is not suitable for the extraction and quantitation of Ca and Fe in soils. The results of this study indicate that the M3 method for the determination of K, Mg, Zn, Mn, and Cu in soils is both accurate and precise when standardized procedures are used. The method has been proven to be suitable for use as a reference method for testing soil materials for extractable P, K, Mg, Zn, Mn, and Cu. Further study may be needed to confirm the suitability of the M3 method for Ca and Fe.