Measurement of total phosphorus and organic phosphorus contents of animal manure composts by the dry combustion method

The objective of this paper was to determine the optimum ignition temperature to measure precisely the total and organic P contents of animal manure composts by the dry combustion method. Total P content of the composts was measured by the wet digestion method and the dry combustion method, while the organic P content was measured by the fractionation method and the dry combustion method. Total P content of the composts ranged from 4.43 to 48.6 g kg⁻¹ and there were significant correlations between the total P content measured by the wet digestion method and that measured by the dry combustion method at 550°C. Organic P content measured by the fractionation method ranged from 0.44 to 8.62 g kg⁻¹. Organic P content measured by the dry combustion method ranged from 0.10 to 8.63 g kg⁻¹, and there were significant correlations (r = 0.951) between the data obtained by the fractionation method and those obtained by the dry combustion method at 550°C. Ignition at 240 and 350°C was insufficient to completely decompose the organic matter, and resulted in the underestimation of the total and organic P contents of the composts. The proportion of organic P in all the compost samples ranged from 0.54 to 46%. Based on these results, we concluded that 550°C was the optimum ignition temperature for a precise determination of the total and organic P contents of animal manure composts by the dry combustion method.