QUOTE (papaduc @ Sep 10 2009, 01:21 AM)

It only needs to be applied once and your plants are sorted. Better to do the job properly.
That would depend on how much loam there is in your compost, if you are using a peat based compost with little or no loam, like Allmix, then the mycorrhizal fungi have a hard time reproducing in that medium and you are advised to reapply at each potting up. If on the other hand you are using a compost that is high in loam the VAM's find that a much more friendly environment and are able to reproduce more easily.
This is mainly because peat is a dead sterile medium, think how long things last without decaying when submerged in peat bogs

. Loam provides a much richer environment where the fungi will be able to obtain all the normal minerals and form association with beneficial bacteria that they'd find in their natural habitat.
It has been suggested that Trichoderma Harzianum will do better in a peat based compost than VAM's as they obtain all their nutrients from the plant roots but do seem to encourage heavier root formation, possibly to extend their living space or because the roots are better protected from disease.