I visited a store last week, out in the country, where people might be expected to know better, and discovered that one of their products was 'organic fertilizer.' That's right; they were bagging cow dung and selling it, at six dollars a pound, no less. And this in a remote farming area surrounded by cow pastures!
"Organic fertilizer" sounds lovely, but truth be told, for a little work, you can get it free from any nearby farmer. Or you can usually pay someone to deliver it for you. It doesn't come in plastic bags, but I guarantee, it will be less than six dollars a pound.
What kinds of manure work best as organic fertilizer? Well, the store had one thing right; cow dung is best. Because cows digest so little of their food, cow manure won't burn plants the way nitrogen-rich chicken manure will. Nor does it need to be composted before putting it on the garden. (Actually, elephant dung is slightly better than cow dung. But this article assumes that you'll have a slightly easier time finding cow dung than elephant dung.)
Horse manure is another good one. It's not quite as nutrient-rich as cow dung, but again, it won't burn plants and it can be applied directly to the soil.
Chicken manure is the best as far as sheer nutrients go. It's rich in nitrogen; an application of chicken manure will grow some of the best corn you can imagine. However, chicken manure is what's know as a 'hot' manure; it will burn plants, and cannot be applied directly to the soil. In order to use it, let it compost for at least three months (six months to a year is better) in an out-of-the-way corner of the garden, then apply it sparingly to your unplanted beds and wait at least two weeks and one good rain before planting in it. Or, mix it with dry matter, such as leaf mold or sawdust, to cut down on the heat. (Unfortunately, this also dilutes the nutrients. But chicken manure diluted this way makes an excellent organic fertilizer because it can be applied directly to the beds, left two weeks and one good rain, and then planted.)
Ultimately, organic fertilizer should be free or--at the very least--cheap as...well...dirt. Because that's exactly what it is.