Olive mill wastewater (OMW) with added N and Mg was used as a medium in a shake-flask, repeated-batch fermentation process with a passively immobilized, acid-producing, strain of Aspergillus niger. The latter was able to reduce the phenolic content of the waste material to 59% of its initial amount and to lower the pH of the medium. Rock phosphate, added to the OMW medium was solubilized to a maximum amount of 0·5 g/l during the fourth batch cycle with a corresponding productivity of 10·6 mg P/l.h. Five types of OMW ± RP, microbially treated or not, were tested in a soil-Trifolium system for their fertilizing ability. The beneficial effect of microbially treated OMW was more evident during the first crop cycle. The best plant growth response and P uptake were found in mycorrhizal plants grown in soil amended with fungal treated OMW+RP. M.F., N.V. and M.V. thank the C.N.R.-Italy and Spanish CICYT for the grants. Peer reviewed