From the Gardening Guru, I couldn't have said it better:
Organic vs. Synthetic Fertilizers
The earth neither grows old or wears out if it is dunged. - Columella, circa 45 A.D.
Organic fertilizers differ from chemicals, in that, they feed your plants while building the soil's structure. Soils with lots of organic material, remain loose and airy, are better able to hold moisture and nutrients, foster growth of soil organisms, including earthworms, and promote healthier root development. Building a healthy soil is the key to successful organic gardening.
Another advantage of organic fertilizers, is that they are made from plant and animal sources, or from rock powders. These materials need to be broken down by soil microbes in order for their nutrients to be released, and that takes time. Because organic fertilizer works slowly, it provides long-term nutrition and steady, rather than excessive growth.
Tip: Looking for
organic fertilizer to ensure a beautiful, healthy crop? The author of this site suggests Planet Natural because of their large selection and great prices.
On the other hand, chemical fertilizers work fast, which is a good thing, if that's what you're looking for. They can make a bad garden or lawn look good much quicker than most organics can. However, it's my opinion that the nutrients are released too quickly, creating a great deal of top growth before the roots are able to catch up. This kind of growth often leads to weaker plants. Also, because they are so rich, synthetic chemicals can easily be over applied and "burn" roots or create toxic concentration of salts. Learn more about the
adverse environmental effects of synthetic chemicals here.
Chemical fertilizers will not improve the structure of the soil. In fact, because they are composed of high concentrations of mineral salts, they are capable of killing off many of the soil organisms that are responsible for decomposition, and soil formation. If only chemicals are added, the soil gradually loses its organic matter and microbiotic activity. As this material is used up, the
soil structure breaks down, becoming lifeless, compact and less able to hold water and nutrients. The result is pretty clear - you'll have to use more and more fertilizer.