By means of adding different amounts of wheat straw into soils, the soil samples were collected after one whole year’s indoor incubation to study effects of soil organic matter (SOM) contents on parameters of phosphorus (P) adsorption and desorption at different soil/water ratios in the calcareous yellow fluvo-aquic soil and lime concretion black soil. Langmuir equations can be used to simulate P adsorption isotherms of the two soils with a series of different SOM contents at the significant levels. For the two soils, the quadratic parabolic curves could well describe the relationships between SOM content and maximum P adsorption capacity (Xm), constant of P adsorption bonding energy (K), maximum buffering capacities (MBC), degree of P adsorption saturation (DPS) and desorbed P concentration at the same soil/water ratio. Before the inflexion of the P adsorption parabolic curve, the P adsorption performance is strengthened, and desorption performance is weakened gradually with increase of SOM content, while the reverse phenomenon occur after the inflexion of the parabolic curve. For each type of soil with a series of SOM contents, the desorbed P concentration has negative correlation with Xm, K and MBC, and positive correlation with DPS at each water/soil ratio. With the increase of SOM contents, soil P bioavailability, loss risk and buffering capacity are effectively modulated by the changes of P adsorption and desorption characteristics at different soil/water ratios. Compared with yellow fluvo-aquic soil, lime concretion black soil has higher clay, calcium carbonate and lower TP, Olsen-P contents, therefore the inflexions of Xm, K, MBC and SOM curves of lime concretion black soil are obviously higher than those of yellow fluvo-aquic soil, while that of DPS of lime concretion black soil is lower.