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United States Department of Agriculture

Agricultural Research Service

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

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

Submitted to: American Society of Agronomy Abstracts
Publication Type: Abstract Only
Publication Acceptance Date: November 2, 2003
Publication Date: November 2, 2003
Citation: Reeves III, J.B., Lugo-Ospina, A., Dao, T.H., Van Kessel, J.S. 2003. Evaluation of quick tests for dissolved phosphorus determination in dairy manures. [abstract]. American Society of Agronomy Abstracts. Annual Meeting Abstracts. November 2-6, 2003. No. A05-reeves921387-P.

Technical Abstract: Accurate and real-time information about phosphorus (P) forms and concentration in animal manure in storage stockpiles is needed because the variability of nutrient concentrations over time and space increase the risk of over-application of nutrients and dissolved (DP) discharges from agricultural lands. We evaluated the suitability of a commercial hand-held reflectometer, a hydrometer, and an electrical conductivity (EC) meter for determining DP and/or total P (TP) in dairy manure suspensions. Bulk samples (n=107) collected from farms across CT, MD, NY, PA, and VA were highly variable in total solid (TS) concentrations, ranging from 11 to 213 g L-1, 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 = 0.29). Inclusion of pH and squared-pH terms improved the prediction of manure DP from RQFlex results (r2 = 0.47). An improved correlation was found for 102 of the 107 samples (RQFlex-DP = 0.947 * MRPs + 509; r2 = 0.64) upon excluding outlier samples of lowest pH with highest MRP. Manure TS concentrations were related to hydrometer readings (r2 = 0.49) that were in turn related to TP (r2 = 0.33), but not to either RQFlex-DP or MRP concentrations. Relationships between suspensions' EC and DP or TP were not particularly strong for this group of dairy manures. Therefore, the RQFlex method is a viable option for on-site quick estimates of DP concentrations in dairy manure that can be made more robust when complemented with pH measurements. Estimates of total P can be derived from specific gravity of the manures. These quick measurement tools can be incorporated into a real-time P sensing system that may enhance near-real time manure management decisions on the farm and the utilization of liquid manures for producing optimal crop responses while minimizing soil buildup and potential for offsite loss of P.

Last Modified: 10/1/2014
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