Phosphorus (P) dynamics are generally considered to be governed by physical and chemical processes including sorption-desorption and precipitation-dissolution; however, biological processes governing organic P mineralisation can also play a significant role in P availability, especially in soils with low available inorganic P. Organic P mineralization has been difficult to measure due to the high reactivity of phosphate anions in soil solution, which are often rapidly sorbed to soil surfaces following their release.  Isotopic techniques using either ³²P or ³³P have been developed to measure P mineralisation under steady state conditions.  In addition to P mineralisation, microbial P turnover can also be measured using isotopic techniques to better understand the role that the microbial P cycling plays in soil P availability.  In this experiment, gross P mineralisation and microbial P turnover is assessed using isotopic techniques in four soils having varying levels of available inorganic P.  The soils were collected from dairy farm forage fields containing Medicago sativa (Alfalfa) and Phleum pretense L. (Timothy-grass) in Ontario, Canada.  Preliminary results support the hypothesis that organic P mineralisation and microbial P cycling may be enhanced under conditions of low available soil inorganic P.  This research is expected to help with the more accurate assessment of total plant available P, something that would be especially useful when assessing soils with low available inorganic P.