Organic fertilizers are made from natural ingredients, most of which are organic in origin and renewable. In addition to items in their natural form, there are also organic fertilizers in pellet and liquid form.
Manufactured Organic Fertilizers
Manufactured organic fertilizers are very similar in use to inorganic fertilizers, right down to the NPK ratios being printed on the box. One difference between these organic fertilizers and standard inorganic fertilizers is the speed at which the nutrients are released. Organic fertilizers are typically slow-release, which reduces the likelihood of the fertilizer burning your plants.
Plants do not like intense bursts of fertilizer followed by no fertilizer for long periods. Too much fertilizer at once can burn leaves and roots. Also, most of it will end up leaking from the soil into water, where it pollutes your groundwater and local water bodies. If you do use non-organic fertilizers, slow release forms are better than standard non-organic.
Organic fertilizers may include just one ingredient, or they may use a mix for more balanced nutrient content. Ingredients often include things like bloodmeal, bonemeal, cottonseed meal, feathermeal, fish, kelp/seaweed, muriate of potash (ashes of plants and trees), urea, rock phosphate, and bat guano. One thing to be aware of with rock phosphate is that it is a non-renewable resource. The others listed above are all renewable.
Different ingredients are not interchangeable. Each has a somewhat different NPK ratio. Urea, for example, has a very high Nitrogen content while guano has a little Nitrogen and a much larger amount of Phosphate. Most people are probably best off getting an organic fertilizer containing a mix of ingredients that produce a fairly balanced NPK ratio.
Fertilizer or Soil Amendment?
The line between organic fertilizers and soil amendments is fuzzy. Manure is both, for example.
Manure is in the grey zone between fertilizers and soil amendments. It is also often the cheapest available fertilizer, providing you can transport it to your garden. You may be able to find it free locally, especially horse manure. People who have horses are often happy to give it to anyone willing to remove it from their property either free or very cheaply.
Fresh manure is best used more than 120 days prior to harvest for plants where the portion eaten touches the soil and 90 days where it does not. The reason for this is that fresh manure can contain E. Coli bacteria. If manure is composted before use this is much less of an issue.
Green manuring means growing a crop that is then turned under to increase soil fertility. Legumes are popular green manures as they add nitrogen to the soil. This is more a soil amendment than a fertilizer.
Compost is also more a soil amendment than a fertilizer, but it does add nutrients to the soil as well as improving soil texture and water retention.