Experimental factors can influence the results of soil P tests. Particularly, the effects of variation in
shaking vessels, centrifugation, and filtration on soil test P (STP) measurements were studied using deionized water (DI), Mehlich
III, Bray, and Olsen extraction techniques for removing P from soil. In vessel comparison experiments, extractions with
Erlenmeyer flasks tended to yield lower STP values (with higher standard deviations) when compared to those performed with
conical tubes. Centrifugation and filtration had the
greatest impact on water extactable P (WEP). Spectrophotometric experiments showed that centrifugation (or filtration
with 0.45 micron filter paper) removed fine soil particles, which scattered the radiation of the spectrometer. During
colorometric analysis, the scattering was perceived as absorption, which led to falsely high STP values. Further experimentation
showed the low ionic strength
of the extract limited the aggregation of soil particles, explaining the scattering in WEP analysis using only qualitative
filtration.
A paper outlining the experiments and results, entitled "Impact of Experimental Factors in Soil Phosphorus Tests: Shaking
Vessel, Filtration, and Centrifugation," was accepted by the journal Communications in Soil Science and Plant
Analysis in September, 2007, and is in press.