Recently, incorporation of crop straw into soils is being largely encouraged worldwide. To explore the possible influence of incorporation of decomposed crop straw on the speciation (i.e., inorganic mercury/IHg, and methylmercury/MMHg) and phytoavailability of mercury, mercury-contaminated farming soil was amended with different amounts (i.e., low, medium or high) of straw organic fertilizer (SF, mainly consisting of decomposed rice straw) or humus (HU) and incubated for a month. Potential phytoavailability of IHg, assessed by CaCl2 extraction, was significantly lower in soils amended with low/medium SF, possibly due to the immobilization effect of SF-organic matter on IHg. In contrast, phytoavailability of IHg was significantly higher in soils incorporated with high HU, possibly explained by the leaching effect of dissolved HU on soil-bound IHg. For MMHg, incorporation of medium/high HU significantly increased MMHg phytoavailability, while SF addition had little effect. Interestingly, MMHg levels in SF/HU amended soils were generally lower than that in soil receiving no amendment, probably because complexation of IHg with SF/HU organics decreased IHg availability to methylation microorganisms. Overall, current results suggested that incorporation of decomposed crop straw may have multiple effects on mercury biogeochemistry in soils, which should be considered when applying SF into mercury-contaminated farming soils.