In 1947 in the former U.S.S.R., phosphobacterin, a soil inoculum, was produced by adsorbing spores of Bacillus
megaterium var. phosphaticum on Kaolinite. This Gram positive bacteria is a PSM mobilizing poorly soluble sources of
organic and inorganic P [30]. For many years phosphobacterin field inoculation trials were performed with different crop
plants and legumes, in different soils and the results obtained varied from no response to 70% yield increase, with an
average of 10%. The best plant response to phosphobacterin was observed in neutral or alkaline soils rich in organic acids.
The outcome of inoculation of maize with PGPR is significantly influenced by the nature of the soil used [31], and this can
be attributed in part to the variability of the indigenous microflora and microfauna which interact with the introduced PGPR.
In fact soil fauna plays an important role in the soil-plant-microbes interactions because grazing of bacteria by a wide
diversity of protozoa affects plant growth by modifying bacterial community structures and by keeping them young and
active, or by acting as concentrators of phytohormones produced by bacteria, or by releasing nitrogen to the plant (for a
review see Antoun and Prévost [32]). Mycorrhizas are also an important factor influencing the interaction between plant and
PGPR, and they can influence the microbial communities structure in the roots since a mycorrhizal plant have root exudates
different than non-mycorrhizal plants [28, 32]. Unfortunately many studies still overlook today the mycorrhizal effect and
several reports presumably on the rhizosphere might be indeed the results of the study of the mycorrhizosphere. Very simple
method can be used to determine if a plant is mycorrhizal or not and to evaluate the importance of root colonization by these
symbiotic fungi [33].