The pollution of waterways and the deterioration of aquatic ecosystems that receive agricultural drainage becomes an environmental concern when excessive amounts of phosphorus (P) are used for crop production.  More conservative use of P while avoiding yield losses can only be achieved through more accurate predictions or assessments of the amount of bioavailable soil-P. Yet those concerned about agriculturally and environmentally available P rely on empirically derived relationships between chemical P extractants and crop response to added fertilizer.  To address this issue, a new soil-P index based on a direct query of soil microorganisms was developed and compared to traditional soil P tests as well as environmentally available based P measurements.  This biological P index is based on activity measurements of gene expression known to be involved in microbial acquisition of non-labile forms of P as well as specific lipid biomarkers.  The specific objective of this study was to compare results from the new biologically-based P index to more traditional measures of soil P availability and crop response to added P.  A greenhouse study was implemented consisting of five rates of P and various soil types using a randomized complete block design. Soil samples were taken from pots and analyzed using traditional agronomic soil P tests such as Mehlich-3, and environmental P tests such as total P, oxalate-extractable P, and algal-available P by sulfur digest.  Soil samples from all pots were taken and analyzed for algal available P and enzyme available P.  Tissue samples were also taken from all pots and analyzed for biomass uptake of P.  These results will be correlated with the microbial biomarker assays of P availability. Data will be presented.