Five tests were used to study the potential of the NSA
isolates to solubilize P namely (1) production of clear zone
on calcium-phosphate or PRP agar media; (2) determination
of the amount of released P; (3) changes in pH in broth
amended with PRP; (4) production of organic acids; and (5)
production of acid and alkaline phosphatases. It is inter-
esting that not all isolates that were able to solubilize P were
positive to all these five tests. This indicates that the
mechanism of P solubilization among isolates could be
different. Although M. endolithica was initially selected based
on its relatively high superior performance in all the five in
vitro tests employed, it is also possible that M. endolithica
may also have helped the roots in the uptake of the
solubilized phosphate by means other than phosphate
solubilization. Activities of M. endolithica may have also directly or indirectly helped to avoid the phosphate solubilized from becoming fixed in the soil.