Fully understanding how soil test P level many change with manure application is important agronomically when one is trying to supply a portion or all of a crop's Phosphorus (P) needs with manure. It is also important environmentally as many nutrient management regulations are based in part on soil test P levels and P balancing/budgeting. Previous research has shown that the availability of manure P may vary with manure source meaning that not all manures will change soil test P by the same amount. This study was conducted to assess the effect of various manure application rates on P availability from dairy slurry, swine slurry, and potassium phosphate. An incubation study was conducted using two soils collected in Michigan: a Colwood loam and an Oshtemo sandy loam. The initial Bray 1-P soil test level were 15 and 11 mg kg^-1 for the Colwood and Oshtemo soils, respectively. Dairy slurry, swine slurry, and potassium phosphate (KH₂PO₄) were applied to 50 g of each soil in triplicate. The rates of total P applied were 0, 10, 25, 50, 100, and 150 mg kg^-1 for all sources with the exception that dairy slurry was not applied at the 150 mg kg^-1 rate and swine slurry was not applied at the 10 mg kg^-1 rate. These rates were chosen because they represent the range of application rates that might commonly be applied on farms, though the highest rate for each slurry is excessively high. The dairy slurry contained 530 mg L^-1 of total P with 79 % as inorganic P. The swine slurry contained 1254 mg L^-1 of total P with 88 % as total P. Treated soils were incubated at 20 C for 6 weeks at a moisture equivalent to 50 % of field capacity of each soil. At the end of the incubation, soils were air dried, sieved, and analyzed for Bray 1-P. The relative availability (RA) of manure P was calculated as the change in soil test P when manure was applied divided by the change in soil test P when potassium phosphate was applied. This was done for each total P application rate. As the rate of total P applied increased, the soil test level increased for all P sources on all soils. However, the rate of change in the soil test level varied between P sources for a given amount of total P applied. The RA of dairy slurry compared to potassium phosphate was 0.71 and 0.79 for Colwood and Oshtemo soil, respectively, when averaged over all rates of P applied. The RA of swine slurry was 0.88 and 0.95 for Colwood and Oshtemo soil, respectively, when averaged over all rates of P applied. When each P application rate is looked at individually, the RA of swine slurry on the Colwood soil increases with P application rate, while the RA of dairy slurry in not affect by P application rate. On the Oshtemo soil, both slurries have an increase in RA with the first two or three rates with no further increase at higher rates. This data shows that the RA of manure P to increase soil test levels compared to inorganic P varies with animal species. The average RA of manure P in similar to the inorganic P content of the manure. However, when individual P application rates are studied, RA changes with application rate, suggesting that the inorganic P content of manure alone is not the sole factor controlling the RA. Other research using these same manures and soils, show that for swine slurry there is preferential sorption of organic P. Thus, as more total P is applied in swine slurry, there may be a greater availability of the inorganic P because of the preferential sorption of the increasing quantities of organic P. In the previous study, preferential sorption of organic P in dairy slurry was not seen, and may explain why there were no real differences in RA at the different P rates for the Colwood soil. Research in the area of manure P availability should take into account application rate factors for different animal species that are typical of the range in rates actually applied in the field. Several past laboratory studies like this have used total P application rates around 100 mg kg^-1, which for many manure sources produces a very high field application rate that may not be typical or allowable under nutrient management regulations. Research on manure P availability should continue to ensure a strong database from which practical recommendations may be made to sustain crop production and the environment.