Using Hydromorphic soil sample (Três Municípios Series) from the Piracicaba region, State of São Paulo, Brasil, a study was carried out to determine the phosphorus content of a solution added to the soil as compared to the quantity of fixed phosphorus. The effect of incubation period was also studied. The relative quantity of fixed phosphorus was calculated, following a scheme previously laid down, from 4 incubation periods (0- 2 - 20 and 200 hours) and 4 concentrations of a monocalcium phosphate solution (0 - 500 - 1,000 and 1,500 P ppm). Each treatment consisting of the equivalent of 10g of oven-dry soil received 4 ml of a solution of an already known concentration. At the end of the incubation period for each treatment, the phosphorus content of stratum was determined using an extraction solution of 0.025 N H2SO4 and 0,05 NHCl. Experimental data showed that relative quantity of fixed phosphorus, under the considered conditions, fullfilled the requirements of this work. Statistical analysis of data showed that an interaction between the studied factors and that the maximum quantity of fixed phosphorus could be reached with around 60% of the longest incubation period observed.