Organic fertilizers are simply those derived from natural plant or animal sources. Just because a fertilizer is labeled organic, though, doesn't necessarily mean that its free from pesticides or genetically modified materials. To be free of such contaminants, the product must be labeled for certified organic use. Soil amendments, such as manure and compost, are not technically fertilizers, according to the Colorado State University Extension, because manufacturers are not required to identify the minimum amount of nutrients in them. However, they improve soil structure and drainage and provide some nutrients, as well.

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Gardeners should weigh the benefits and drawbacks of all fertilizers.

Cottonseed Meal

Cottonseed meal is high in nitrogen and breaks down slowly over one to four months. It is generally applied as a granular product at a rate of 10 lbs. per 1,000 square feet of soil. Many pesticides are used when growing cotton, and cottonseed meal often contains pesticide residues. Most are genetically modified.

Corn Gluten

Corn gluten is derived from corn by-products and is applied to the soil as granules. As it breaks down, corn gluten releases a high level of nitrogen. It also forms a thin barrier over the soil, making it an effective pre-emergent weed control. It is used primarily on lawns, but can also be used in vegetable gardens after desired plants have emerged. Corn gluten releases nitrogen for one to four months. Typical application is 20 to 40 lbs. per 1,000 square feet of soil.

Fish Emulsion

Fish emulsions are made from the waste products of fish and are usually applied in a liquid formula. Fish emulsion provides nitrogen and several micronutrients and generally costs less than other animal-based fertilizers, such as blood meal and bone meal. Fish emulsion has an offensive smell, but the scent dissipates within a few days. Fish powders and fish meal are more expensive than fish emulsion, but they also provide more nutrients.

Seaweed

Kelp is available in liquid, powder or granular formulas. Rather than nitrogen, potassium or phosphorus, these products provide micronutrients and growth hormones that improve plant growth and overall health.

Animal By-Products

By-products of slaughter house production include bone meal, blood meal and feather meal. Blood meal has a very high nitrogen content and can burn plants if improperly applied. Bone meal has a high phosphate content, although the phosphate may not be available to plants when soil pH levels are above 7.0, according to Colorado State University Extension. Feather meal products provide good nitrogen levels, but break down slowly. All animal by-product fertilizers are relatively expensive.