Tests used to assess phosphorus (P) status in manure amended calcareous soils are often inaccurate and underestimate the amount of potentially mobile P. This can lead to over applications of manure and an increase in mobile P which potentially threatens the quality of receiving waters. We examined P dynamics in calcareous systems in order to understand and develop a test that would accurately predict the fate of P in these environments. This included batch and kinetic studies using mineral phases found in calcareous soils such as calcite, brushite , and the iron oxide ferrihydrite. Humic acid obtained from the International Humic Substances Society was used as an analogue for manure. Our hypothesis was that the soluble organic matter from the manure inhibited the sorption of P and consequently influenced how tests such as the Olsen's bicarbonate extract or a calcium chloride extract gave misleading predictions of bioavailable P. Although soluble organic matter did have some effect on P sorption kinetics, from initial results it appears that the form of inorganic P in the manure source is mostly responsible for the unexpected test results. Further work will use geochemical modeling to verify our findings and assist in developing an accurate P indicator test.