Unfortunately the vast majority of agars contain carbohydrates that can
be used by all and sundry. This might be just enough to mess up what you
are trying to do.
If you doubt it, try plating out a soil suspension (or your favourite
inoculum) on water agar, and when a few colonies appear flood the plate
with iodine. No starch was added but you will find that there are areas
of clearing which means that SOMETHING that was iodine reactive was
removed. OK ?
AKA <P.J.Harris at reading.ac.uk>