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.
Manufactured organic fertilizers include compost, bloodmeal, bone meal and seaweed extracts. Other examples are natural enzyme digested proteins, fish meal, and feather meal.
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, are fully organic in the sense of organic chemistry. In fact, 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. For one thing organic growers typically produce some or all of their fertilizer on-site, thus lowering operating costs considerably. Then there is the matter of how effective they are at promoting plant growth, chemical soil test results aside. The answers are encouraging. Since the majority of nitrogen supplying organic fertilizers contain insoluble nitrogen and are slow release fertilizers their effectiveness can be greater than conventional nitrogen fertilzers.
Implicit in modern theories of organic agriculture is the idea that the pendulum has swung the other way to some extent in thinking about plant nutrition. While admitting the obvious success of Leibig's theory, they 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:
* Mobilizing existing soil nutrients, so that good growth is achieved with lower nutrient densities while wasting less
* Releasing nutrients at a slower, more consistent rate, helping to avoid a boom-and-bust pattern
* Helping to retain soil moisture, reducing the stress due to temporary moisture stress
* Improving the soil structure
Organics also have the advantage of avoiding certain long-term problems associated with the regular heavy use of artificial fertilizers:
* the possibility of "burning" plants with the concentrated chemicals (i.e. an over supply of some nutrients)
* the progressive decrease of real or perceived "soil health", apparent in loss of structure, reduced ability to absorb precipitation, lightening of soil color, etc.
* the necessity of reapplying artificial fertilizers regularly (and perhaps in increasing quantities) to maintain fertility
* the cost (substantial and rising in recent years) and resulting lack of independence
Organic fertilizers also have their disadvantages:
* As acknowledged above, they are typically a dilute source of nutrients compared to inorganic fertilizers, and where significant amounts of nutrients are required for profitable yields, very large amounts of organic fertilizers must be applied. This results in prohibitive transportation and application costs, especially where the agriculture is practiced a long distance from the source of the organic fertilizer.
* The composition of organic fertilizers tends to be highly variable, so that accurate application of nutrients to match plant production is difficult. Hence, large-scale agriculture tends to rely on inorganic fertilizers while organic fertilizers are cost-effective on small-scale horticultural or domestic gardens.
* Improperly-processed organic fertilizers may contain pathogens harmful to humans or plants. Organic fertilizers are derived from natural sources, which may include animal feces or plant/animal matter contaminated with pathogens. However, proper composting of raw materials used in organic fertilizers will kill pathogens.
In practice a compromise between the use of artificial and organic fertilizers is common, typically by 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.
There is an important difference between fertilizers that are 'organic' in origin and those fertilizers approved for use by organic certification bodies for use in organic farming and organic gardening. Some of their approved fertilizers may be inorganic, naturally occurring chemical compounds, e.g. minerals.
