Chabot et al. [6] reported that PSM present in four bulk soils from the province of Quebec, with contrasting available P,
ranged from 2.8 to 3 x106 cfu g-1 (colony forming unit per g of dry soil), and they represented from 26 to 46% of the total
soil microflora. To determine if there is any potential correlation between soil physicochemical properties and the incidence
of PSM, Bouchard [7] studied 20 different soils used for potato production. PSM represented from 4 to 30% of the total soil
microflora and their number ranged from 2 to 30 x 107 cfu g-1 of bulk soil. The number of PSM found in soil will greatly
depend on the sparingly soluble P source used, which is often a di or tri-calcium phosphate. In fact, out of 287 PSM isolated
from Quebec soils on CaHPO4, 53% and 67 % were able mobilize P from AlPO4 and FePO4, respective [7]. Although soils
contain an important number of PSMs, these microorganisms mobilize enough P to cover their own needs, but not in excess
to supply the plant [8]. It is important also to note that many free living microorganisms are not necessarily adapted to life in
the mycorrhizosphere.