Water extractable P (WEP) in biosolids and manures has been shown to be an effective indicator of runoff P loss potential. To be an effective commercial test, a WEP method must meet reproducibility and other production laboratory analytical criteria, and it must reflect differences in runoff dissolved P. The objective of this project was to develop data for choosing a single WEP procedure for inclusion in a national (CSREES SERA-17) testing manual. Cooperating laboratories conducted extraction experiments to evaluate (i) solution:solids ratio (10, 100 or 200) and (ii) the sample size used to determine WEP. A runoff box study was also conducted to evaluate the effect of different WEP protocols on the correlation between dissolved P in runoff and WEP of surface applied biosolids and manures. As extraction ratio increased from 1:10 to 1:200, so did WEP, with a few exceptions. All WEP extraction protocols were highly correlated (r2 > 0.7) with dissolved P in runoff. WEP extractions were reproducible within labs and between labs. Increasing manure or biosolids sample size for WEP test did not improve precision. Obtaining a 1:10 extract suitable for ICP or colorimetric determination was difficult for some biosolids and dairy manure samples. The consensus extraction procedure will be included in a revision of "Methods of Phosphorus Analysis for Soils, Sediments, Residuals, and Waters" available at www.sera17.ext.vt.edu.
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