Evaluating the Influence of Storage Time,Sample‐handling Method,and Filter Paper on the Measurement of Water‐Extractable Phosphorus in Animal Manures

Abstract

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 3×3×2 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.