ARS | Publication request: INTERLABORATORY COMPARISON OF SOIL PHOSPHORUS EXTRACTED BY VARIOUS SOIL TEST METHODS

Submitted to: Communications in Soil Science and Plant Analysis
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: September 1, 2001
Publication Date: September 30, 2001
Citation: Kleinman, P.J., Sharpley, A.N., Gartley, K., Jarrell, W.M., Kuo, S., Menon, R.G., Myers, R., Reddy, K.R., Skogely, E.O. 2001. Interlaboratory comparison of soil phosphorus extracted by various soil test methods. Communications in Soil Science and Plant Analysis. 32(15&16):2325-2345.

Interpretive Summary: Common soil phosphorus analyses can be consistently replicated. The increasing use of soil phosphorus data from various sources raises concern about the repeatability of results between laboratories. This study subjected 24 soils from across the U.S. to a variety of soil phosphorus analyses in order to assess interlaboratory variation in soil P methods. Results indicate that comparison of Olsen P data from multiple labs has th greatest potential for error, while Mehlich-3 and Bray-1 results are consistently replicated between labs. Results indicate that some common soil test P protocols, when carefully conducted, yield data that may be reliably compared, such as in the compilation of regional and national soil databases.

Technical Abstract: The widespread use of soil P data, either in the context of agronomic or environmental management, requires an explicit understanding of potential errors related to soil P testing. This study compares a variety of soil P extraction methods, each performed by 9 separate laboratories on 24 soils from across the U.S. Clay ranged from 0 to 47%, pH from 4.2 to 8.6, and Mehlich-3 P concentration from 2 to 205 mg kg-1. Average interlaboratory coefficients of variation (CVs) ranged from 0.11 to 0.22 for solution extracts (Bray-1 P, Fe-strip P, Mehlich-3 P, and Olsen P) and from 0.11 to 0.70 for saturated paste extracts (resin capsules and resin membranes, incubated for 2, 4 and 7 days). For soil tests based upon solution extracts, Olsen P had the greatest variability among laboratories (CV = 0.22), despite its reputed suitability for a wider range of soils than Bray-1 and Mehlich-3. Soil test data were highly correlated, with the lowest correlations occurring between Olsen and Bray-1 P or Olsen and Mehlich-3 P (r = 0.77 and 0.84, respectively) and the highest correlations occurring between Olsen P and Fe-strip P or Mehlich-3 and Bray-1 P (r = 0.94 for both correlations). Results indicate that some common soil test P protocols, when carefully conducted, yield data that may be reliably compared, such as in the compilation of regional and national soil databases.