Oh, man. I'm sure to catch flack for messing up my title and admitting I am a school teacher and rightfully so! Sorry about that. I wish I knew how to fix it!

No worries -- it happens. You have to see your mistake BEFORE you hit the "Submit Message" button.

I think you have some misinformation; plant's roots CAN and DO rot in a hydroponic environment. In fact, root rot and damp-off (similar bugaboo) are among hydroponic growers' greatest challenges. It varies with each cultivar (type of plant), but root rot and damp-off each stem from a common problem: Lack of available free oxygen. Damp off actually caries with it some other causes, but when there's plenty of oxygen available, damp off has a hard go of it.

Hydroponic growers (myself included) do several things to "aerate" the roots. Personally, I use air injection by way of a fish-tank-type air pump, hose and air stones. This bubbling adds free oxygen to the water and keeps the roots healthy just like it does fish.

Another method is to add diluted hydrogen peroxide to the nutrient mix. This adds oxygen on more of a "chemical" basis. The byproducts of hydrogen peroxide breakdown are oxygen and water.

"Drip" systems use the natural flow of water through an aggregate medium, the end result being trickling, which imparts free oxygen to the recirculating nutrient solution.

Hope this is enough information for you to teach your kids.

God bless.

:O)

You hit the nail right on the hammer.

Root rot is a big problem in any hydroponic system if it's not properly maintained. It's just less of a problem typically because the people running a hydroponics system tend to be a little more precision-oriented and thus less likely to overwater (which is generally what causes the problem in soil.)

Most home gardeners using soil are notorious for overwatering, over fertilizing, and other forms of "loving their plants to death". It's just hard to break ourselves of the belief that if a little water is good, more is better. If a little fertilizer is good, more is better.

But most hydroponic systems automate the watering process (or in the case of DWC and NFT, do so continuously) so it's much harder to "overwater". The problem comes, as freemangreens said, when the water (nutrient solution) isn't properly aerated. The roots require oxygen so if they're constantly inundated with the nutrient solution it has to contain enough oxygen to keep them healthy.

High oxygen levels also have the benefit of discouraging harmful microbial growths that can hurt the plants several different ways.

H2O2 is one way to oxygenate water, but I think of it as an emergency method more than anything else. Direct air injection (bubbling air with aquarium pumps and airstones or something similar) is very common, I use that. You can also agitate water to aerate it by pumping it through sprayer heads or pretty much any situation where it splashes or breaks into droplets as it falls. (Smooth flow doesn't aerate well.)

Flood and Drain systems aerate the roots by forcing existing air up out of the medium when the nutrient solution rises up, then drawing fresh air down as it drains away.

Drip systems tend to generate a capillary action that draws air in, as well as operating with liquid droplets small enough to easily absorb available oxygen.