Page Banner

United States Department of Agriculture

Agricultural Research Service

Title: Evaluation of Quick Tests for Dissolved Phosphorus Determination in Dairy Manures

Authors
item Lugo-Ospina, Ancizar - USDA, ARS, AMBL
item Dao, Thanh
item Van Kessel, Jo Ann
item Reeves Iii, James

Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: September 8, 2004
Publication Date: January 5, 2005
Citation: Lugo-Ospina, A., Dao, T.H., Van Kessel, J.S., Reeves III, J.B. 2005. Evaluation of quick tests for dissolved phosphorus determination in dairy manures. Environmental Pollution. 135:155-162.

Interpretive Summary: There is need for a method for the rapid determination of dissolved phosphorus (DP) in manures so that farmers can avoid excess application of phosphorus to avoid leaching and run off. The objective of this study was to evaluate the suitability of several potential methods for DP determination on the farm. These included a hand-held device called a reflectometer (RQFlex) which uses small test strips, hydrometers which determine density, and measurements of electrical conductivity (EC) and manure total solids (TS) concentrations. Results were compared to a standard laboratory procedure. The manure samples (n = 107) were highly variable in composition as measured by any of the methods described. On a gravimetric (weight) basis, the RQFlex reflectometer yielded lower DP estimates that the laboratory standard method and were only poorly related to the laboratory values, although better results were obtained after removal of the 5 samples containing the highest levels of DP. Low manure suspension pH values were associated with high DP, especially for the laboratory method. The prediction of manure DP from RQFlex results, on a volumetric basis, required the inclusion of this pH and squared-pH (pH*pH) in the predictive equation. Manure TS or hydrometer readings were not related to either RQFlex-DP or DP concentrations as measured in the laboratory. In conclusion, experimental results indicate that only the RQFlex method is a viable option for on-site measurements of dissolved P concentrations in dairy farm manure, and even then only a rough estimate is possible.

Technical Abstract: The objective of this study was to evaluate the suitability of a hand-held reflectometer, hydrometers, and measurements of electrical conductivity (EC) and manure total solids (TS) concentrations for determining dissolved phosphorus (DP) in dairy manure suspensions, and to compare the estimated DP concentrations to the molybdate-reactive P (MRP) concentrations obtained in the laboratory using the molybdate-ascorbic acid method. The manure samples (n = 107) were highly variable in TS, ranging from 11 to 213 g L-1 and in suspensions, pH (6.3 to 9.2) and EC (6.2 to 53.3 dS m-1). On a gravimetric basis, the RQFlex reflectometer yielded lower DP estimates that were related to molybdate-reactive phosphorus (MRP) concentrations as follows: RQFlex-DP = 0.471 * MRPs + 1102 (r2 = 0.293). An improved correlation was found for 102 of the 107 samples (RQFlex-DP = 0.947 * MRPs + 509; r2 = 0.637) upon excluding samples of highest MRP. Low manure suspension pH values were associated with high DP, especially for the MRP method. The prediction of manure DP from RQFlex results, on a volumetric basis, required the inclusion of pH and squared-pH in the predictive equation. Manure TS or hydrometer readings were not related to either RQFlex-DP or MRP concentrations. An effort made to determine MRP by near-infrared spectroscopy indicated that no relationship existed with MRP. In conclusion, experimental results indicate that only the RQFlex method is a viable option for on-site measurements of dissolved P concentrations in dairy farm manure, and even then only a rough estimate is possible.

Last Modified: 4/19/2014
Footer Content Back to Top of Page