Potting compost. Bulb fibre retains far too much water for use where it will get rained on.

I've just been to our local Garden Club meeting on putting plants in tubs and containers. One tip I picked up was to add grit to the compost for drainage and also stand the pot on pot feet.
And I thought I knew what to do!

Bulb fibre contains little goodness, if any as bulbs contain their own energy and most indoor ones will be of little use after flowering. It does contain charcoal which keeps indoor compost sweet. So you can use bulb fibre indoors but compost is the one to use for outside.

Ive never added anything just plant them in soil that other plants have been in, they come up year after year, never been disappointed with the flowers.

plant in non-clumping clay type cat litter.
retains moisture;
excess drains away;
neutral;
inert;
re-usable;
allows air to the plants roots, essential for healthy root growth;
does not compact over time;
easy to weed and keep weed free;
easy to tell if plant needs watering, just stick a BBQ skewer into the cat litter, if it's wet/damp do not water, if it's dry water;
lighter weight than compost;
looks more attractive too;
has a light harmless pleasant smell or no smell at all (some brands add a perfume, don't worry, it's harmless to people and plants).
hope you find this helpful.