Measuring water-extractable phosphorus in manure as an indicator of phosphorus in runoff

Water-extractable P (WEP) in manure is correlated with P concentration in runoff from soils amended with manure and is, thus, an effective indicator of environmental P loss. This study sought to elucidate methodological factors affecting WEP measurement in manure and to quantify errors related to two established methods of manure WEP measurement. Dairy cow (Bos taurus) manure, poultry (Gallus gallus domesticus L.) (layer) manure, and swine (Sus scrofa domestica L.) slurry were used. Varying dry matter/distilled water ratios (1 to 20:200) revealed that greater dilution of manure dry matter increased WEP (mean 1.8-5.4 g kg^-1), likely because of the dissolution of calcium phosphates. Increasing shaking time from 1 min to 24 h, increased manure WEP concentration (average 3.7-8.2 g kg^-1). Filtration with Whatman 1 paper filters resulted in significantly higher WEP measurements in dairy and poultry manure (4.1 g kg^-1) than with a 0.45-μm filtration (3.7 g kg^-1). No significant difference was observed in the swine slurry. A rainfall-runoff experiment using simulated rainfall was conducted to determine the effect of the individual factors on predicting dissolved-reactive P (DRP) concentration in runoff. Comparison of regression coefficients relating manure WEP to runoff DRP concentration revealed an optimum shaking time between 30 min and 2 h, but did not support any single manure/distilled water ratio or filtration method. Replication of two established methods of manure WEP measurement resulted in coefficients of variation of 0.01 to 0.12. Results of this study support the use of a single method with a fixed manure/distilled water ratio for liquid and dry manures.