Comparison of the Methods for Quantifying Organic Phosphorus in Turkey Litter Compost

Various methods for quantifying and characterizing organic P in turkey litter compost (TLC) were compared. The standard extractant used, both for the NaOH-Ethylenediaminetetraacetic acid (EDTA) method measuring organic P and for solution 31P NMR spectroscopy, is a mixture of 0.25 M NaOH and 0.05 M Na2EDTA, however these methods underestimated organic P in TLC compared with the ignition and the sequential fractionation methods. Approximately 83% of the total organic P in TLC was extracted with a weak acid extractant, ammonium acetate (pH 4.2), by the sequential fractionation method. Acid soluble phytates, such as Ca(Mg)-phytates, usually present in animal manure composts would be insoluble at pH above 6.0. These organic P fractions can be important sources of plant available P, however, neither direct quantification nor solution 31P NMR spectroscopy of organic P extracted by the NaOH-EDTA would be able to detect them. To avoid underestimation of organic P in animal manure composts, modifications to either the concentration of EDTA or the compost to solution ratio may be necessary. These changes would allow for complete chelation of the Ca2+ and Mg2+ present in animal manures and composts. Otherwise, an analysis of organic P in both alkaline and acid extracts by solution 31P NMR spectroscopy may be necessary.