There is an abundant literature showing positive, neutral or negative effects on plant growth and yield, in response to
inoculation with PSMs. This illustrates the complexity of all the interactions taking place in soil between plants and their
symbiotic mycorrhizal fungi partners, and the different microbial communities associated with AM fungi or plant roots, and
fauna particularly protozoa and nematodes grazing on microorganisms. To overcome the problem of variability of the
results and to develop effective microbial inocula for the improvement of plant growth and P-nutrition, the following points
may be considered.