One of the best fertilizers for a garden of any kind (or lawn!) is well composted manure from chickens. If you can't or don't want chickens and don't know anyone who has them, you can buy it, but here's a frugal way to get the same benefit.
Chickens and birds' manure is basically the same, so if you have birds around (and who doesn't?) you can encourage them to come and stay in a certain areas of your yard by providing food and water. It won't be long until the ground under these provisions will be splattered with bird droppings. That's gold for your garden! Rake or sweep up the area and put the droppings, along with soil, leaves and so on, into a compost bin or a marked hole in the ground and let it compost. If you put it in the ground and leave it, it will turn into rich compost within a month or so. A compost bin, regularly turned, will take less time.
Of course, this will make only a tiny bit of compost, since you will only have a tiny bit of manure, but if you want to make more at one time, scoop up the droppings and store them in a dry place until you have enough. You might be surprised at how quickly it adds up!