INRA-CMSE, UMR Microbiologie des sols-Géosol BP 86510, 21065
DIJON Cedex, FRANCE.
E-mail : catroux@dijon.inra.fr
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A
critical analysis of the problems associated with the selection and practical
use of PSB (Phosphate Solubilizing Bacteria) is presented taking into account,
as "reference experiences", the practical use of Biological Nitrogen
Fixing (BNF) organisms and biocontrol agents (BA).
To inoculate a
micro organism successfully in a soil or in a rhizosphere requires :
· to have an
efficient strain able to realize the claimed effect in field conditions
· to have the
technology to produce an efficient inoculant with a good shelf-life
· to have a
convenient method of inoculation delivering the required number of cells at the
right place and in an active form.
· To get a good
survival of the inoculated microorganisms in order to obtain a prolonged
effect.
All these points
will be discussed with the published results on PSB, in order to propose
possible improvements in the development of their practical use in agriculture.
To get efficient
strains requires to have a more ecological strategy of selection than generally
performed until now. Several points must be emphasized and considered to
improve PSB selection. As for other beneficial micro organisms, the classical
scheme of selection is first to isolate strains, to screen them in the
laboratory with in vitro tests, to check after for their efficacy with
biotests and finally to test them in the reality of the field.
Isolation is the
first point to improve in considering more carefully the ecology of the niche
to be colonized (temperature, carbon and other nutrients availability,
especially P…).
Screening process
performed in vitro is risky as numerous examples have demonstrated the
absence of correlation between in vitro tests and efficacy in
biotests. The main point to improve is the selection for competitiveness which
is never considered in the first steps of the screening, despite the fact that
an efficient but non competitive strain will never give the expected results in
the field or will not be reliable
As the vehicle of
inoculation is the inoculant, a technology is needed to produce the culture, to
formulate it to get a good shelf life of one year ore more as the lower cost as
possible. Therefore, unrealistic rates of inoculation reported in research
works must be considered carefully when a new inoculation is to be assessed for
future developments.
Method of
inoculation are needed and considering the experience of BNF and BA uses,
emphasizes to design new methods adapted to the case of PSB.
The survival of the
micro organisms in soil or rhizosphere after inoculation is of main importance
for a prolonged action and eventually for the establishment in soil. For PSB,
reported data remain still scarce despite the fact survival is an important
point for the efficacy of inoculation.
In conclusion, the
priority for research will be discussed, considering the reported data on PSB
allows to develop inoculants in a near future with good chances of commercial
application for use in well defined conditions.