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Phosphorus supply to eucalypts is deficient in many Brazilian soils and must be augmented with fertilizers to promote economic growth of plantations. Diffusion is generally considered to be the major mechanism of P fertilizer movement in these types of soils; therefore, we studied the effect of P fertilizer source and time of incubation of fertilizer with soil on the P diffusive flux in four Oxisols. Five soluble P sources [Ca(H2PO4)2, NH4H2PO4, (NH4)2HPO4, Na2HPO4·2H2O, and K2HPO4] were applied to the soils at a rate of 500 g P m‐3soils. Microscope slides were covered with anion‐exchange paper (filter paper impregnated with Fe hydroxide) and laid between two soil layers in a diffusion chamber. Fifteen days later, P was extracted from the exchange paper and analyzed. Five hundred grams P m‐3soil as the Ca source were also applied to soil samples and incubated for 30 d. Phosphorus diffusive flux was measured as described above. The Ca source of P reduced P diffusive flux by as much as 77% compared with the other sources. This reduction was attributed to the effect the P source had on soil pH. Sandy soils showed less difference between P sources than did clayey soils because the P applied to the sandy soils was in excess of their maximum sorptive capacity. Incubation for 30 d reduced P diffusive flux by up to 81%. Different P sources affect the availability of P in these soils by influencing mobility. Choosing the best source can increase diffusive flux by up to 242% under these soil conditions.