Laboratory Characterization of Phosphorus in Fresh and Oven‐Dried Organic Amendments

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Abstract
This study was performed to determine the forms of P and to examine the influence of oven‐drying on P forms in different organic amendments. Samples of biosolids, beef and dairy cattle manures, and hog manures from sow and nursery barns were used in this study. Both fresh and oven‐dried amendments were analyzed for inorganic (Pi), organic (Po), and total phosphorus using a modified Hedley fractionation technique. Water extracted about 10% of total biosolids P and 30 to 40% of total hog and cattle manure P. The amount of P extracted by NaHCO3 ranged from 21 to 32% of total P in all organic amendments except in the dairy cattle manure with 45% of total P. The labile P fraction (sum of H2O‐ and NaHCO3–extractable P) was 24% of biosolids P, 60% of hog manure P, and 70% of dairy cattle manure P. The residual P was about 10% in biosolids and cattle manures and 5 to 8% in hog manures. Oven‐drying caused a transformation in forms of P in the organic amendments. In hog manures, H2O‐extractable Po was transformed to Pi, while in the dairy manure NaHCO3–extractable P was converted to H2O‐extractable Pi with oven‐drying. Therefore, caution should be exercised in using oven‐drying for studies that evaluate forms of P in organic amendments. Overall, these results indicate that biosolids P may be less susceptible to loss by water when added to agricultural land.