I have been reading up on this lately, and I feel surrounded by things I can make liquid fert from now, I don't think I'll ever be buying fertilizer again
1. Comfrey - Get a comfrey bush and strip the leaves from it, (1/2 a kilo of leaves) and cover in a 10L bucket water, the water will go a very dark colour, best to cover it, as it will smell a bit when it is ready to use, but is great when you dilute it down, (1 part comfrey liquid : 25 parts water) and spray it on your garden as a liquid feed. It contains lots of potassium (very important for flower and fruit growth), calcium, trace elements as well as Vit B1, B2, B3, B12
Confrey does tend to pop up and wander, so if you don't have a large area for it to grow in, put it in a pot. Comfrey is also GREAT in your compost heap, it helps to 'activate' it. I used to every now and then strip my bush back to a few leaves, and just scatter them evenly on the compost heap and then gently water if the heap was dry.
2. Seaweed - This is one of natures super liquid fertilizers. Gather seaweed where you can find it, as many different types as possible as they all contain different varieties of goodies, then wash it well to get rid of the worst of the salt. Place in a bucket or barrel and cover with water, and leave to steep until it turns a dark colour, you can smell it, and it starts to bubble a little.
Dilute the liquid 1:10 and spray over plants or just water with it.
Seaweed fertilizer also will help your plants to be less prone to frosts.
3. Dandelion tea - You can gather dandelion plants and boil them up to make a 'tea' which can then be cooled and used to water over your plants.
Any others to suggest?