Development of a water-extractable phosphorus test for manure: An interlaboratory study

The loss of P in runoff from agricultural land to which manure has been applied is related to water extractable P (WEP) in manure. However, a standard method to routinely measure WEP in manures has not been established and variables impacting the measurement have not been widely studied. In this investigation, the impact of manure holding times (1-22 d), WEP extract holding times (0-17 d with and without acidification), and method of P measurement on WEP were evaluated. In addition, four manure samples (one dairy, one swine, two poultry) and proposed WEP method were distributed to seven public and private laboratories to assess inter- and intralaboratory variability of WEP test results. The proposed method entailed extracting manures with water at a 1:200 ratio (manure solids/water), shaking for 60 min on a reciprocating shaker, and either filtering or centrifuging before P measurement by inductively coupled plasma atomic emission spectroscopy (ICP). Results show refrigerated (4°C) manure samples can be held up to 22 d and acidified extracts 18 d before analysis. Separation procedures (filtering vs. centrifuging) did not impact WEP measurements. While the method of P measurement (ICP vs. colorimetric) did have a significant impact on test results, the two methods were highly correlated and results within 5 to 10%. The proposed manure WEP method shows a high level of precision (relative standard deviations [RSDs] < 6.5) within laboratories, although greater variability (RSDs of 13.3-28.8) exists among laboratories when compared with other standard manure analyses, such as total P.