Submitted to: Soil Science Society of America Journal
Publication Type: Peer reviewed journal
Publication Acceptance Date: 12/1/2004
Publication Date: 5/20/2005
Citation: Wolf, A.M., Kleinman, P.J., Sharpley, A.N., Beegle, D.B. 2005. Development of a water extractable phosphorus test for manure: An inter-laboratory study. Soil Science Society of America Journal. 69:695-700. Interpretive Summary: Growing concern over the effect of land applied manure on water quality has led to widespread changes in management and regulatory approaches to livestock agriculture. Prudent management of land applied manure requires consideration of the fraction of manure phosphorus that is readily available to be transported by runoff. This study presents the development of a novel manure test for commercial laboratories that measures water extractable phosphorus, a key indicator of runoff available phosphorus. Results show that the new test is robust, easily adapted to commercial laboratory settings and can be replicated with the same degree of confidence as other common manure tests.
Technical Abstract: The relationship between water extractable P (WEP) in manure and P in runoff from agricultural land to which manure has been applied has been clearly demonstrated. However, a standard method for the routine measurement of 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 to 22 days), WEP extract holding times (0 to 17 days with and without acid addition), and method of P measurement on WEP were evaluated. In addition, 4 manure samples (1 dairy, 1 swine, 2 poultry) and the proposed WEP method were distributed to seven public and private laboratories to assess inter- and intra-laboratory variability of the WEP test results. Results show that refrigerated (4ºC) manure samples can be held for an extended period (22 days) before analysis, as can acidified extracts (18 days). Separation procedures (filtering vs centrifuging) did not impact WEP measurements. While method of P measurement (ICP vs colorimetric) did have a significant impact on test results, the two methods were highly correlated and results were within 5-10% of each other. The proposed WEP method for manures shows a fairly high level of precision (RSDs < 6.5) within labs, although greater variability exists among labs for this procedure when compared to other standard manure analyses.