Fe-siderophore complex is more stable compared to that of Ca-siderophore (Albrecht-Gary and Crumbliss, 1998 and Ams et al., 2002). For example, Fe3+-complex of trihydroxamate siderophore desferrioxamine-B (DFO-B) has a 1:1 stability constant of 1031 while that of Ca complex is only k = 102.6 (Martell et al., 2001). This was consistent with the lower P solubilized by bacterial spent media from phosphate rock than FePO4 (Table 1 and Fig. 2). The data also suggested that siderophore played a more important role than organic acids in solubilizing FePO4 than phosphate rock.