this is untrue- a plant needs 17 minerals to grow. these minerals come from soil or from fertilizer applied to the soil when growing in a soil-based medium. when growing hydroponically you must account for these nutirents by adding them in chemical form instead of allowing the plant roots to "find" them in the dirt. the plant needs in the most quantity C H & O, it obtains these through air and water H20 and CO2. next is N P K- your average dirt fertalizer will add these. next in order of importance are Calcium, Magnesium, & Sulfur. then there are Fe, Zn, Cu, B, Mn, Ni, Mo, & Cl. the plant takes these nutirents up in the same way as it would in soil only they are more readily available because the hydroponic "system" delivers them to the roots instead of the roots having to search for them. since the plant is absorbing the same nutrients in the same way as with dirt, there should be no harmful chemicals, or anyhting else for that matter. i remeber a post from a while back claiming that hydroponics caused vegetables with high levels of radioactivity. people often want to believe that hydroponics somehow manipulates how the plant grows but in truth it just is a different way of providing what a perfect soil would provide. air, water, support, and food. hope this answered your question. hydroponics accually grows vegetables with a higher nutritional value because of the optimum growing conditions. ~Ryan

Actually, you could probably say there hasn't been much work on the safety of any food crops until someone gets sick.

A few potential health problems you might have with soil-grown plants should be absent in true hydroponics in which roots are in solution, and there is no solid root medium.

Lead is sometimes a problem in leaf vegetables like lettuce or root crops because of lead uptake or lead-contaminated soil adhering to the produce.

http://apha.confex.com/apha/130am/techprogram/paper_50440.htm

Human diseases also should be less prevalent in hydroponic vegetables or fruits because they are usually grown in greenhouses. The U.S. Center for Disease Control estimates 5% of food poisonings are caused by fruits and vegetables. Most of these cases are probably due to raw vegetables contaminated with microbes from manures.

http://www.kgh.on.ca/kgh/Infection%20Control/foodpois.htm

There may be less pesticide residues on hydroponic crops because they are grown indoors. Although if the greenhouse is not run efficiently, pesticides may be needed even on hydroponic crops.

I thought this was an interesting post so I am bumping it up. I recently came across an online 1998 abstract on nitrates & lettuce. They found winter growth & low light conditions resulted in higher nitrate concentrations in lettuce. A lower nutrient concentration was recommended to compensate for this. This is only an abstract, & the specifics of the study  sample size, statistics, etc. are not mentioned.

http://www.caes.state.ct.us/PlantScienceDay/1998PSD/PSDfield.htm#9

Go to:
9. NUTRITION OF HYDROPONIC LETTUCE

Where did you "hear" this? I think hydroponics is less likely to contain unknown toxins than organic due to the fact the materials are quantified and tested rather than guess and by golly, which is what organics typicly consists of. Of course with either method water and air, with organic add soil, are all possible carriers of toxins. Organics depends on inputs from infinately variable souurces of unknown and typicly untested content. Most inputs into hydroponics have had at least some testing as to actual content. To add to the risks consider soil is typicly host to several potentially deadly organisms such as botulism, which are more likely to be absent with hydroponics. Some hydro is organic, but as of yet very little. Both methods have merit, organics is probably one of the best ways to recycle many waste stream items. Chemical fertilizing enables production of produce when organic materials are not locally available. Hydroponics give great yeilds for the space, are typicly the most convienient to tend and harvest by hand labor, are typicly water conserving, and not dependant at all on soil fertility or even existance allowing production right on top of rock sand etc. Personally in a hungry world i see all production systems to have their place.

This is a problem relating to winter growing of leafy greens in greenhouses, not necessarily the hydroponic aspects. The problem has been mitigated by removing all nutirents for the last 24-48 hours and the levels drop.

The problem of nitrate accumulation in lettuce is due to the physiology of the lettuce  not hydroponics. In the abstract mentioned above they found winter growth & low light conditions resulted in higher nitrate concentrations in lettuce. Hydroponics does, however, allow one more control in decreasing nitrate levels. The above link recommends decreasing the nitrate concentration by 1/3 during winter growth. Nitrate accumulation can also occur in tropical/semitropical soil based plants that are grown in winter.

Some studies suggest supplementary lighting may also nitrate levels:

http://www.actahort.org/books/559/559_48.htm

HereÂs another abstract of a study on nitrate & media:

http://www.actahort.org/books/548/548_60.htm