It all depends on what is meant by organic. Technically, organic fertilizers contain carbon, and inorganic fertilizers do not contain carbon. However, in popular usage fertilizers are referred to as organic if the nutrients in the product are derived either from the remains of living organisms or their by-products. Under this definition, blood meal and manure are examples of organic fertilizers.

Urea contains carbon and is, therefore, considered an organic fertilizer. However, it is a synthetic organic fertilizer, an organic product manufactured from inorganic materials. Consequently, using the popular usage definition, it is not an organic fertilizer and cannot be used for organic crop production.

Some additional examples of organic fertilizers include: alfalfa hay, bone meal, compost, cotton seed meal, fish emulsion, lawn clippings (recycling your lawn clippings may supply one-third to one-half of the annual nitrogen requirement of the lawn), and composted and activated sewer sludge.

Some advantages of an organic fertilizer include: has low burn potential; has relatively slow release; contains micronutrients; conditions the soil; adds organic matter to the soil; and, with some, can raise the pH level. Some disadvantages include: emits an odor (e.g., sewer sludge, fish emulsion); contains weed seeds (e.g., compost, some manures such as horse manure); is bulky, making them difficult to handle and distribute; and may contain heavy metals (e.g., some sewage sludge).

Another important disadvantage of the organic fertilizers listed above is the low level of the three essential nutrients: nitrogen (N), phosphorous (P), and potassium (K). For example, activated sludge has an analysis of 6-3-0, while composted sludge (mostly used as a soil amendment) has an analysis of 1-2-0.