Soil to grow food on th e balcony

You can buy compost soil (terreaux) via Bruxelles Proprété (www.bru-compost.be)

Places like Brico have a variety of soils including some specifically developed for vegetable gardens. Or go to any garden center.

If you buy compost it will have a set level of nutrients in it, vegetables
Maybe very greedy and use that up quickly then not thrive. So it simply requires that you also purchase a suitable liquid fertiliser that you mix into the water and feed regularly them to improve the amount of food available and the yield. Places like bricco sell this. Make sure you obtain general compost not that for say acid loving plants or cactus
Happy growing

I just notice herbs in your post herbs hate rich soil and tend to prefer dry conditions so add on grit to make the soil less rich for my herb pots I use the cheapest compost they sell and mix in grit lettuce is the opposite