What are the advantages and disadvantages of using organic versus inorganic or synthetic fertilizers?

Organics typically cost more per unit of nutrient than synthetics, vary widely in their nutrient content, but often contain a large assortment of plant nutrients in small amounts. Also it is usually assumed that organic fertilizers are slowly released. They are more slowly released and made available to plants than many inorganic fertilizers, but release varies among organic sources. In warm soil conditions found in much of California, the nitrogen release can be completed after as little as six to eight weeks. The principal limitations of organic fertilizers are their bulk, availability, odor, potential salt and weed seed hazards, and expense per pound of nutrient. Organic concentrates—such as bonemeal, cottonseed, and fish emulsion — contain a variety of nutrients, but their cost per pound of nutrients is high relative to inorganic fertilizers. The value of manures and organic concentrates does not lie solely in their nutritional value, however. Some organic fertilizer materials ...  more