Evaluating the influence of storage time, sample-handling method, and filter paper on the measurement of water-extractable phosphorus in animal manures

Surface-applied manures create a potential phosphorus (P) runoff hazard, especially when unincorporated. In such cases, the concentration of water-extractable P in the manure has been correlated to soluble P concentrations in runoff. This study evaluated the influence of holding time, sample-handling procedure, and filtration method on measurement of the water-extractable P content of manures in a 3 x 3 x 2 factorial arrangement of treatments. A two-way interaction between holding time and sample-handling procedure occurred for most samples. Six samples had water-extractable P concentrations that were less than or equal to dried and dried/ground treatments. Only one sample had higher water-extractable P concentrations for fresh than for dried and dried/ground treatments. When significant differences occurred as a result of the filtration method, results for Whatman No. 40 filters, with a larger pore size than 0.45 μm nitrocellulose membranes, were usually higher. There was no significant difference in the coefficient of variation across sample-handling procedures, suggesting that efforts to dry and/or grind samples were not needed. These results support the adoption of a standardized protocol for measuring water-extractable P in manures that represents the appropriate balance between the ease of implementation and the strength of the correlation to P runoff concentrations.