Naturally occurring organic fertilizers include manure, slurry, worm castings, peat, seaweed, sewage , and guano. Green manure crops are also grown to add nutrients to the soil. Naturally occurring minerals such as mine rock phosphate, sulfate of potash and limestone are also considered Organic Fertilizers.
The decomposing crop residue from prior years is another source of fertility. Though not strictly considered "fertilizer", the distinction seems more a matter of words than reality.
Some ambiguity in the usage of the term 'organic' exists because some of synthetic fertilizers, such as urea and urea formaldehyde. It would be difficult to chemically distinguish between urea of biological origin and that produced synthetically. On the other hand, some fertilizer materials commonly approved for organic agriculture, such as powdered limestone, mined rock phosphate and Chilean saltpeter, are inorganic in the use of the term by chemistry.
Although the density of nutrients in organic material is comparatively modest, they have some advantages. The majority of nitrogen supplying organic fertilizers contain insoluble nitrogen and act as a slow-release fertilizer. Although it's widely thought organic fertilizer is better, balance responsible use of organic or inorganic fertilizers can be just as good for the soil.
Modern theories of organic agriculture admit the obvious success of Liebig's theory, but stress that there are serious limitations to the current methods of implementing it via chemical fertilization. They re-emphasize the role of humus and other organic components of soil, which are believed to play several important roles:
Organics also have the advantage of avoiding certain problems associated with the regular heavy use of artificial fertilizers:
Organic fertilizers can have disadvantages:
In non-organic farming a compromise between the use of artificial and organic fertilizers is common, often using inorganic fertilizers supplemented with the application of organics that are readily available such as the return of crop residues or the application of manure.