This is an interesting point and worthy of discussion. There is, and has been, a huge debate over whether or not hydroponics is organic. Then government agencies got involved and it became more of a lobbying effort because, let's face it, the ability to place an organic label on your goods makes them worth more.
So I have used a more simple definition, this is just me talking now but I have looked at some research backing this up. To me, the fact that hydroponics is not ever cultured in soil is irrelevant, to me, the word organic hinges on whether or not herbicides or pesticides were used is everything. If you took a tomato grown hydroponically and a tomato grown organically and prepared them each for chemical analysis, they would be identical and the analyst likely could not differentiate. But if there were pesticides or herbicides used in either, that would stand out.
When it comes down to the nutrients used in a hydroponic system, the plant needs, to pick one ingredient, nitrogen. It will extract the same chemical from soil or from a hydroponic nutrient blend and the result is the same. With a properly maintained nutrient the plant can't tell the difference. Similarly, soil that is not properly maintained will be depleted of nutrients and the plants will suffer in an unbalanced soil as well. Although plants can't talk, they can communicate their deficiencies or excesses to us by our observation. Excesses or deficiencies of certain nutrients or trace elements will show up on the leaves or fruit if out of balance. This is true for soil and for hydroponics.
All this being said, there are suppliers who sell organic nutrients for hydroponics. They are pricey and usually made from plant extracts, similar to what Magus described above but a bit more controlled. Bottom line is if you want 100% organic then nutrients are available to float your boat.
By growing hydroponically, indoors or in my greenhouse, I can control the growing media and always start with fresh disease free heat sterilized media and I can clean out my system and sump between crops to minimize disease. This, along with germinating everything I grow myself so I do not introduce disease from suppliers, greatly eliminates the need for chemical treatment.
Ironically there is a product called neem oil which is a natural pesticide which organic gardeners can use and still claim the organic label. Neem oil is a natural source of a chemical called Azadirachtin which nature provides to Neem trees. It is extracted from the seeds and made into a concentrate. This concentrate is applied to plants to provide "natural" protection against insects and usually prevents insects from laying eggs on plants treated. But, and this is a big but, it is not naturally in this concentration but still is allowed to fly the organic flag. Following the same logic, chemicals used to mix nutrients are also from natural sources. For example, epsom salts is a commonly used salt in hydroponic nutrients. It is actually magnesium sulfate and it is found in mines and limestone caves. To be sold as epsom salts it is usually refined by dissolving and recrystallizing the salt to make it more pure. This refining process is a big no-no for those who claim it's not organic, but is it all that different than refining neem oil? The organic not organic debate is full of examples like these. As I said, it's more capitalism than anything. That's why I draw the line at pesticides and herbicides.
That's my 2¢ about the organic or not debate.
