Water soluble packing peanuts are made from cornstarch. I would bet that, as
fast as I've seen them dissolve in water, they would be next to useless in a
pot--they'd dissolve the first or second time they were watered...
As to drainage...it is, I know, a common belief that putting sand/gravel in the
bottom of pots will increase drainage. This is untrue. In fact, by filling
part of the pot with an alternate medium, one effectively creates a shallower
pot...the water will tend to pool at the soil medium/drainage medium interface.
A deeper pot will provide better drainage, as will making a faster draining
soil medium (mix perlite/sand/bark chips with the medium). You can see this
drainage effect in a rectangular sponge. Saturate a sponge and first hold it
horizontally, then vertically. The vertical sponge will lose more water, and
the remaining saturated area will remain in a smaller percentage of the
vertical volume. This is what happens in a deeper pot. This is also why
azeleas are often put in shallow pots...they have lots of surface roots and
need the less-efficient drainage (higher water table, if you will) of the
shallower pot.
Kelly
former horticulture student (including greenhouse production)
currently studying landscape architecture at UC Berkeley