Very interesting line-up. Even though it lacked scientific rigour, it still gives an indication how the different mediums treat the roots.
As a long time user of both rockwool and hydrotones in NFT, and now soon two years of growing in coco as well, I've done all three, but never in a side-by-side.
To start with, in NFT, the rockwool and hydrotones are simply used as starting mediums, a root propagator for the young clone or seedling. The roots then spills out on the NFT trays and into the reservoir, which is the real medium.
I've also used a Flo-Gro with hydrotones though, which is a re-circulating drip system a la Waterfarm.
After experimenting with different coco mixes, I went 100% coco, using them in pots either top fed by hand, or bottom fed on trays. The principle here is run to waste, or constant flow.
I do believe that if you had gone constant drip or flow with the hydrotones, you would have seen a whole different result in terms of growth. Hydrotones is a intert medium, meaning that it has close to zero absorption capacity (while the aerating capacity is great). The whole principle behind an ebb and flow system is of course to supply a nutrient solution to an absorbing medium, that holds on to the liquid and distributes it to the root system. So the root system in the hydrotones are dependent on water pockets, spreader mats and spreading out on the trays in order to find liquid.
I can see how the rockwool waterlogged the medium, but not the coco. While coco has a great absorption capacity (8-9 times its own weight), the coarse coir fibres still provides sufficient aeration for roots even when totally saturated. I've only seen waterlogging in 100% coco coir when the coco was re-cycled (fibers degraded) and the root system was underdeveloped. I've run coco coir pots bottom fed on constant flow trays, without a hint of waterlogging.
Rockwool is also an inert medium, but thanks to its porosity it manages to retain liquid, sometimes a little too well. Young plants beware. Its Caption Exchange Capacity (the capacity to 'hold on' to certain nutrients and later re-distribute them) is zero, just as water, while coco has a tendency to hold on to certain nutrients (initially at least), such as calcium and potassium.
Rockwool therefore holds an advantage over coco if the nutrient (solution) flow is constant, while coco holds an advantage in an ebb and flow system.
Therefore, the medium most suited for this kind of system/feeding is coco, rockwool less, and hydrotones even less.
Set it up differently, and you'll get different results.
Originally Posted by KissOfDeath
As you can imagine, the coco stayed quite wet. As a result, the root system was not forced to expand in search of water and nutrients. It didn't even fill the entire 2 gallon bucket - the bottom 2 or 3 inches had virtually no roots!
Let me expand a little on this. Roots do not grow in a dry medium. In a wet medium they will grow towards the water/mineral source (higher concentration of water and nutrients), always. They will therefore always invest the most saturated parts of the medium, and if they don't, it's either because they couldn't, or because they didn't need to.
One thing worth talking about is the CEC capacity of coco, allowing it to store/concentrate nutrients only to release them to the roots later. A well-saturated coco coir medium therefore - it seems - allows roots to feed more efficiently on a smaller surface than peat soil (lower CEC) or rockwool/hydrotones (zero CEC).
So, rather than saying that the roots didn't expand (in coco) because the medium was too wet, let's say that you were using too big pots, because with coco you can reduce the size and get the same results, the proof being that your coco plant outdid the others with a smaller root system.