Here is an incredible fertilizer you can make for your garden: comfrey tea brew.
If you can grow comfrey, there are a multitude of uses for your garden. In addition to being a fertilizer, it acts as a "dynamic accumulator" meaning it actually mines nutrients from the soil.
This plant is a terrific compost activator. Put some comfrey in the compost heap to add nitrogen and help to heat the heap.
A two-inch layer of comfrey leaves placed around a crop will slowly break down and release plant nutrients, so consider comfrey as a mulch or side dressing.