Posted: 19/05/2014 at 16:11
Actually, it is in some respects dependant on which herbs you want to grow. Some will require a really well drained soil and others will be happier with a more moisture retaining medium.
Anyway, whichever it is you decide on, you will need some sort of drainage holes in the base so that you don't end up with a stagnant medium when it buckets it down (as it is bound to do at some point I guess ). Depending on the actual depth of the bunker, you could use a good loam based compost mixed with a little MPC and with added grit if you want to do the Med type herbs. The "green" herbs won't be too worried if you forgo the added grit
It is worth remembering that any container will require some fiddling about after a couple of years........eg....you remove the top few inches and replace with new medium........simple enough with your Med types. For the annual green herbs, you just replant every year.
I hope this makes some sense