I'm guessing that the majority of allotment holders have tried their hand at making their own plant food at one time or another. If you haven't, here's how I do mine with nettles and now's a great time to get some going....
You'll need; a bucket (preferably with a lid), nettles, water, a house brick and a peg.
I use an empty fertiliser bucket with lid - this one contained fish blood and bone.
It's best to use the youngest, juciest nettles you can find. I tend to pick off just the tops, leaving the old woody bits behind (think PG Tips). The leaves are best 'bruised'. You can tear them with your hands (gloves may be useful) or, the easiest way is to run them over with the lawnmower. Then shove all the bits into your bucket.
All you need to do now is fill the bucket with water. Rainwater would be best - I just shove it under the water butt and fill it up. Then, stick the brick on top, to keep the nettle leaves submerged(ish).
Put the lid on and place the bucket in a nice sunny spot to ferment. It's probably best to take it to the allotment at this stage. Leave it for 3-4 weeks and it's done.
I then strain off the liquid through a riddle into another bucket. The peg comes in handy here as it really REALLY hums.
The pulp can go onto the compost heap. The liquid is your feed. Add a small scoop to your watering can full of water when ready to feed (I use those small plastic scoops that often come with the fertiliser bucket).
I was a bit sceptical when I first tried this but I had some cabbages one year that looked a bit tired and yellow. Within 3 days of feeding with the nettle food they looked strong, heallthy and green. Now, there wasn't a control (maybe I should have just fed half) but I believe it worked. You can use it around the stems, or over the plant leaves.
Good luck if you give it a go!