John D. Toth, Zhengxia Dou, James D. Ferguson, Christine Wang, and Charles F. Ramberg, Jr. University of Pennsylvania School of Veterinary Medicine, 382 West Street Rd., Kennett Square, PA 19348
Measurement of soluble phosphorus in animal manures is an important component in estimating potential environmental losses of P. However, the results may differ depending on how the samples were handled after on-farm collection. Manure samples can be stored under refrigeration for only a relatively short time because of contamination by mold and bacteria growth. Longer-term storage requires either drying or freezing. A number of studies have compared manure P solubility of fresh and dried samples, however, less is known about the effects of sample freezing. We collected freshly-excreted feces from nine commercial dairies (four cow fecal samples per farm). The samples were split into subsamples which were extracted after different handling methods: (i) fresh: extraction performed within 4 hrs of collection, (ii) after 3 and 5 days refrigeration at 5 ºC, (iii) after being frozen at -20 ºC (two freezing-thawing cycles). Extraction was in triplicate in deionized water or 0.1% HCl with a sample dry matter:extractant ratio of 1:300. Soluble inorganic P was measured in filtrates by the phosphomolybdate colorimetric method. Inorganic phosphorus concentrations ranged from 0.8 to 5.8 g P/kg manure in water extracts and from 1.0 to 6.8 g P/kg in HCl extracts. Soluble P in extracts of freshly-excreted manure were generally not significantly different from extract P after 3 or 5 days refrigeration (P=0.05). Analysis of thawed, frozen samples showed a mean increase in soluble inorganic P of 58 and 25% for water and HCl extractants, respectively, compared to fresh or refrigerated manure; however, the degree of increase was not consistent across farms and not all differences were significant at the 5% level. Formation of ice crystals and disruption of cell walls in the frozen samples is likely the cause of the increase in soluble P.
