Here is a little perspective from Brian Dunning at Skeptiod
Scientifically, the term "organic food" is meaningless. It's like saying a "human person". All food is organic. All plants and animals are organic. Traditionally, an organic compound is one produced by life processes; chemically, it's any carbon-containing molecule with a carbon-hydrogen bond. Plastic and coal are organic, a diamond is not. So when we refer to organic food in such a way to exclude similar foods that are just as organic chemically, we're outside of any meaningful scientific use of the word, and are using it as a marketing label.
The biggest misconception is that organic farming does not use fertilizer, herbicides, or pesticides. Of course it does. Fertilizer is essentially chemical nutrient, and the organic version delivers exactly the same chemical load as the synthetic. It has to, otherwise it wouldn't function. All plant fertilizers, organic and synthetic, consist of the same three elements: nitrogen, phosphorus, and potassium. Referring to one as a "chemical" and implying that the other is not, is the worst kind of duplicity, and no intelligent person should tolerate it.
The difference between the two is the source of the chemicals. To make the high-volume commercial versions of both organic and synthetic fertilizer, the source materials are processed in factories and reduced to just the desired chemicals, and the end product, these days, is virtually indistinguishable. Small organic farmers, and home organic farmers, might use fish meal, bone meal, bat guano, or earthworm castings. These are fine products and do indeed deliver the required nutrients. They're just not useful for high volume farming because they're (a) far too expensive, and (b) contain too much ballast, or inactive ingredient, that the crops don't use and merely increase the energy requirements of moving and delivering them.
To make synthetic fertilizer, we start with nitrogen, which we extract from the atmosphere. This process is infinitely sustainable and produces no waste. The potassium is mined from ancient ocean deposits. The phosphorus we get from surface mining of phosphate rock. Although we have centuries of reserves of phosphate rock and millenia of reserves of potassium salts, mining is not sustainable, as these reserves will eventually run out. So, increasingly, producers are turning to seawater extraction for both. This forms a completely sustainable cycle, as the oceans are the ultimate destination of all plant matter and farm runoff.
http://skeptoid.com/episodes/4166