Experimental Factors that Impact Results of Water Extractable Phosphorus in Soils.
Daniel Ebeling and Joel Davis. Wisconsin Lutheran College, 8800 W Bluemound Rd, Milwaukee, WI 53226
In the analysis of Mehlich III, deionized water (DI), Bray, and Olsen soil P extractions, variations in shaking vessel, centrifugation, and filter paper result in different soil test phosphorus (STP) results. None of the differences were as significant as the impact of centrifugation on the results of the water extractable phosphorus (WEP), that is, the extraction done using deionized water. There was up to a five-fold difference in the soil test phosphorus (STP) when an extraction protocol involving centrifuging followed by filtration was compared to a protocol with only filtration. The relative standard deviations in the results were much lower when centrifugation was employed. Various types of filter paper were evaluated for the resulting STP. Further spectrophotometric experiments demonstrated that the centrifugation removed fine soil particles from the extract. Without this step, these particles scatter the radiation of the spectrometer. During a Murphy Riley colorimetric analysis, this scattering of radiation will be perceived as absorption. This misperception results in a falsely high STP. In WEP analysis without centrifugation, the scattering appears to be very significant due to limited aggregation of soil particles in the low ionic strength extract.