At first sight “organic hydroponics” appears to be an oxymoron. There is considerable interest by consumers world-wide to purchase organic vegetables, but a considerable antagonism by organic producer councils to accept that it is possible to use hydroponic systems and still conform with the organic philosophies.
In fact by using nutrient solutions derived from acceptable organic sources, and using biological pathogen control measures, and recirculating hydroponic systems, a more sustainable system of crop production is possible than by using a “classical” soil based organic systems.
People who have some sort of problem associating hydroponics with organics should stop reading this right now and go put their heads back in the sand. The word hydroponics can be loosely interpreted from Latin to mean "working-water" (not "non-organic" as some people seem to think), and since water is allowed for organic growers, hydroponic growers can of course adapt their systems to be fully, or semi-organic.
I don't plan to rant much further, but I have run into many organic exclusionist idealist fervour-spewing radicals who refuse to categorise any hydroponics methods as possibly being organic off hand, without any consideration.
Now on to the more enjoyable technical aspects of organic hydroponics.
Organic substances are defined as those substances obtained directly from Mother Earth without chemical alteration or processing, and including those produced by plants or animals.
A completely organic hydroponics system must use only organic materials for the substrate or media in which the plants are intended to grow, and must utilise only organic fertilizers to supply the necessary nutrients for the plants to grow and flourish.
The growing media can be soil (steam or heat sterilized preferably to prevent disease or fungal infestation), vermiculite, perlite (pearlite), peat, coco-husk (coir), straw, or other organic material(s). I'm not sure whether stone-wool is classified as organic by the organic classifiers, but seeing as it is just physically treated (heated and spun) rock from the Earth, I think it should be, although I don't use or praise it anyway, but some people have been convinced to like it (fanatically in some cases).
Organic fertilizers come from a vast array of sources including bat guano; sea-bird dung; sea-weed ("strip-mined" or "clear0cut" from the oceans); fish entrails and bones; cow, pig, sheep, chicken manure, bones, blood, and urine; and a myriad of other places.
The main problems with using organic fertilizers are nutrient completeness and control issues as well as contamination problems. Nutrient variations between batches and sources of organic nutrients make fetilizer cycling much more difficult than with conventional "chemical" solutions, causing increased runoff into the environment in some cases where ion imbalances occur. Fertilizers such as manure are also sources for possible outbreaks of E. coli bacteria when crops are not processed properly.
When using organic nutrient solutions in hydroponics it is best to discard waste effluent by using it to fertilize ornamental or outdoor crops to minimize environmental damage