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 w ith almost all commonly used soil P extractants, but few tests for the effects of extractants itself on the indicator species have been repo rted. 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 c onducted over a range of extract volumes to determine indicator specie s stability and to identify factors affecting it. Three soil P extract ants (Mehlich-3, Bray-I, and Modified Truog) and three soils were test ed at different levels of extract volume, solution pH, and P concentra tion. In our tests, Bray-I and Modified Truog did not interfere with t he stability of the indicator species under conditions commonly used f or soil P determinations. Mehlich-3 caused fading of the blue color at low P concentrations and precipitation of a blue compound at high P c oncentrations. This instability was most pronounced when Mehlich-3 ext ractant volumes exceeded 2 mL in 50 mL final solutions with P concentr ations greater than 0.4 mg/L. The EDTA and NH4NO3 contained in Mehlich -3 both contributed significantly to the observed instability. Our res ults indicate that use of the Murphy and Riley method with untested ex tractants can produce unreliable P determinations, especially when soi l extract volumes are high relative to the total volume of the colored solution.