The precipitation of insoluble Ca-P phases is the predominant process that reduces P availability to plants in calcareous soils. Previous works highlighted that: I. Ca ion activity in the liquid phase is the mainly responsible for the formation of insoluble Ca-phosphate mineral phases; II. the extent of P-insolubilization is a function of the soil moisture content and the organic matter addition to soil reduces P- insolubilization. The aim of this work is to investigate the effects of organic matter on the phosphorus dynamic in calcareous soils; for this purpose P retention isotherms were carried out on calcium carbonate samples. Calcium carbonate, infact, controlling the Ca concentration in soil solution and in the soil exchange complex, plays a key role in Ca-P precipitation. CaCO3 samples (0.5 g.) dispersed in 40 ml di H2O were treated with different amounts of organic carbon extracted from peat. These systems kept at 25°C were, then, put in contact with different P amounts and after 24 hours total P ions and total and free calcium ions in solution were determined. Diffuse Reflectance Infrared Fourier Tranform (DRIFT) spectra of the lyophilized liquid phases of the different isotherms were also recorded. The results obtained pointed out that organic matter is able to preserve soluble larger amounts either of P ions or of calcium ions. P isotherms on calcium carbonate were, then, carried out in presence of simple organic acids used as surrogates to represent and to explain the organic matter behavior. The results were compared to those obtained in presence of EDTA, a strong Ca ion complexant. Also organic acids were, in different level, effective in avoiding calcium phosphate precipitation, in less extent than organic matter, but in greater extent than EDTA. The Drift spectra revealed that, in presence of organic matter and of some acids ,the absorption region of carboxylate and phosphate ions has changed. These findings and the isotherms analysis suggested us the formation of a ternary complex where calcium ions act as bridges between organic matter and phosphate ions preserving P from insolubilization. In presence of EDTA Ca ions are strongly linked to EDTA and the formation of a ternary complex is impossible.