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TECHNICAL REPORTS

Waste Management

Selection of a Water-Extractable Phosphorus Test for Manures and Biosolids as an Indicator of Runoff Loss Potential

^a USDA-ARS, Pasture Systems and Watershed Management Research Unit, University Park, PA 16802
^b Dep. Crop and Soil Sciences, Oregon State Univ., Corvallis, OR 97331
^c Agricultural Analytical Services Lab., Pennsylvania State Univ., University Park, PA 16802
^d Agricultural and Biological Engineering, Pennsylvania State Univ., University Park, PA 16802
^e School of Veterinary Medicine, Univ. of Pennsylvania, Kennett Square, PA 19348
^f USDA-ARS, National Soil Tilth Lab., Ames, IA 50011
^g USDA-ARS, Northwest Irrigation and Soils Research Lab., Kimberly, ID 83341
^h Dep. of Crop and Soil Environmental Sciences, Virginia Polytechnic Univ., Blacksburg, VA 24061
ⁱ USDA-ARS, Poultry Production and Product Safety Research Unit, Fayetteville, AR 72701
^j Dep. Crop, Soil and Environmental Sciences, Univ. of Arkansas, Fayetteville, AR 72701
^k Dep. Soil and Water Science, Univ. of Florida, Gulf Coast Research and Education Center, Wimauma, FL 33598
^l Dep. Plant and Soil Sciences, Univ. of Delaware, Newark, DE 19716
^m Dep. Biological and Agricultural Engineering, Univ. of Arkansas, Fayetteville, AR 72701
ⁿ Dep. Plant and Soil Sciences, Oklahoma State Univ., Stillwater, OK 74078
^o Environmental Health, Greenhouse and Processing Crops Research Center, Agriculture and Agri-Food Canada, Harrow, ON, Canada N0R 1G0. Mention of trade names does not imply recommendation or endorsement by USDA-ARS

^* Corresponding author (Peter.Kleinman{at}ars.usda.gov).

Received for publication October 13, 2006. The correlation of runoff phosphorus (P) with water-extractable phosphorus (WEP) in land-applied manures and biosolids has spurred wide use of WEP as a water quality indicator. Land managers, planners, and researchers need a common WEP protocol to consistently use WEP in nutrient management. Our objectives were to (i) identify a common WEP protocol with sufficient accuracy and precision to be adopted by commercial testing laboratories and (ii) confirm that the common protocol is a reliable index of runoff P. Ten laboratories across North America evaluated alternative protocols with an array of manure and biosolids samples. A single laboratory analyzed all samples and conducted a separate runoff study with the manures and biosolids. Extraction ratio (solution:solids) was the most important factor affecting WEP, with WEP increasing from 10:1 to 100:1 and increasing from 100:1 to 200:1. When WEP was measured by a single laboratory, correlations with runoff P from packed soil boxes amended with manure and biosolids ranged from 0.79 to 0.92 across all protocol combinations (extraction ratio, filtration method, and P determination method). Correlations with P in runoff were slightly lower but significant when WEP was measured by the 10 labs (r = 0.56–0.86). Based on laboratory repeatability and water quality evaluation criteria, we recommend the following common protocol: 100:1 extraction ratio; 1-h shaking and centrifuge 10 min at 1500 x g (filter with Whatman #1 paper if necessary); and determining P by inductively coupled plasma–atomic emission spectrometry or colorimetric methods.

Abbreviations: ICP, inductively coupled plasma • P_color, phosphorus determined by colorimetry • P_ICP, phosphorus determined by inductively coupled plasma–atomic emission spectrometry • RSD, relative standard deviation • TP, total phosphorus • WEP, water-extractable phosphorus