POTENTIAL ERRORS IN THE USE OF THE MURPHY AND RILEY METHOD FOR DETERMINATION OF PHOSPHORUS IN SOIL EXTRACTS

The Murphy and Riley method for phosphorus (P) determination is used with almost all commonly used soil P extractants, but few tests for the effects of extractants itself on the indicator species have been reported. In practice, it is important to know the period of stability of the indicator species and the range of P and extractant concentrations that produce reliable results. In this study, a series of tests was conducted over a range of extract volumes to determine indicator species stability and to identify factors affecting it. Three soil P extractants (Mehlich-3, Bray-I, and Modified Truog) and three soils were tested at different levels of extract volume, solution pH, and P concentration. In our tests, Bray-I and Modified Truog did not interfere with the stability of the indicator species under conditions commonly used for soil P determinations. Mehlich-3 caused fading of the blue color at low P concentrations and precipitation of a blue compound at high P concentrations. This instability was most pronounced when Mehlich-3 extractant volumes exceeded 2 mL in 50 mL final solutions with P concentrations greater than 0.4 mg/L. The EDTA and NH4NO3 contained in Mehlich-3 both contributed significantly to the observed instability. Our results indicate that use of the Murphy and Riley method with untested extractants can produce unreliable P determinations, especially when soil extract volumes are high relative to the total volume of the colored solution.