Agriculture Reference
In-Depth Information
Fig. 1.2
Isolation and selection of P-solubilizing microorganism from different sources
to formulate a defined medium to elucidate the role of microorganisms in P
mineralization. On the contrary, the NBRIP medium has several advantages over
other media (Nautiyal
1999
). For instance, the NBRIP medium can be used as a
defined medium because it excludes the use of yeast extract. Secondly, NBRIP is
more efficient in a broth assay compared to the Pikovskaya medium.
Despite the variation in the effectiveness of different media, first of all, PSM is
isolated from soils/any source using serial plate dilution method or by enrichment
culture technique (Fig.
1.2
). The serially diluted rhizospheric or non-rhizospheric
soil samples are then spread (100
l) inoculated on solid
Pikovskaya plates or any plates containing insoluble P (e.g. tricalcium phosphate)
and incubated. After proper incubation of inoculated solid Pikovskaya plates for
5-7 days (bacteria) and 3-5 days (fungi and actinomycetes) at 28
μ
l) or streaked or spot (10
μ
2
C, the
P-solubilizing microbes are detected by the formation of clear halo around their
colonies (Plate
1.1
). The development of a clear zone around the colony on the
culture plates are taken as an index of P solubilization. However, the reliability of
this halo-based technique is questioned as many isolates in other studies did not
produce any visible halo/zone on agar plates but could solubilize insoluble in-
organic P in the liquid medium (Gupta et al.
1994
; Louw and Webley
1959
). The
phosphate solubilizers are then maintained on medium, for example, Pikovskaya,
which is used for PSM isolation until use. Since P-solubilizing organisms exhibit
many-fold variations in P-dissolving activity (Khan et al.
2007
) and instability with
regard to their P-solubilizing activity (Illmer and Schinner
1992
), they are repeat-
edly subcultured to test the persistence of P-solubilizing potential. Once the effi-
cient PSM are selected, they are tested for their ability to solubilize insoluble P
