Make your own feed
If you've made a compost bin like mine then you'll be able to siphon off a liquid feed. Alternatively, if you've got access to nettles or comfrey you can use the leaves to make a very nutritious feed for your windowbox:
1) Simply pick your leaves and squash them into a plastic soft drink bottle with the bottom cut off.
2) Up-end it on to the top of a second bottle and tape together with duct tape or other waterproof tape.
3) Place a stone on top of the leaves and leave for at least three weeks for the black liquid to ooze through into the bottom bottle.
4) For larger quantities you can rig up a section of drainpipe in a similar fashion.
Dilute your liquid feed in a 1:10 ratio with water and drench over your windowbox.
Make sure you rinse any crops well before eating them!