Dont quote me on this but the only way oxygen can enter the roots is by being dissolved in water. Roots hanging in air cant make use of it.
No, they can absorb O2 directly from the air as well. The trick is to NOT let the roots dry out.
I running nft tubes and want to increase oxygen to the roots.
If you are aerating your rez, and not pushing too much water thru the system, then you are about as good as it gets unless you flood the tube with O2.
NFT's special thing that sets it above systems like DWC is the thin film of water on the roots. NFT done correctly doesnt even need the rez aerated really.
The thing to know is that O2 dissolves in water slooooowly. The top most, very very thin layer of water that is directly exposed to the air will reach 100% DO very quickly - BUT - that O2 takes a much longer time to penetrate into the deeper parts of the water.
An ideal NFT system takes advantage of that by keeping the roots covered in the thinnest possible layer of water at all times. Remember the F in NFT stands for FILM - not flood. If you flood the roots, your not doing NFT properly.
The other thing NFT does well is it increases the total surface area of the water exposed to the air. The rate of O2 up-take is directly proportional to the surface area exposed to the air. Thats the single thing all forms of aeration have in common - they increase the total surface area of water exposed to the air per unit of time. Bubbling, fluming, waterfalls, etc all work the same way and are pretty much equal in effectiveness. They take water that isnt exposed to the air, and move it so it is exposed.
Other hydro techniques also use this thin film aeration technique. Flood and drain and LPA both work best as far as oxygenation during the off or drain time when only a thin layer of water is left on the roots. The roots will be getting less O2 during the flood time.
HPA and AA take it a step further than any of them by keeping that layer at its thinnest al the time, plus the micro sized droplets achieve 100% DO almost as soon as they leave the nozzle.
That thin layer works because it can replenish the O2 the plant uses as fast as it goes into the root. Remember, the thin layer reaches 100% DO very quickly, so that thin layer of water will always be at 100% DO even while the roots are taking up oxygen. If it is a thick layer, then the O2 doesnt get replenished as fast and the DO will drop.
Having said all that, the differences are minute. DWC roots are submerged 100% of the time and the roots and plants do just fine. Flood/drain, LPA, NFT HPA, AA are all small steps up as far as aeration/oxygen delivery. IF you do everything else 100% right, you might see an improvement in yields by increasing the aeration, but its not going to be huge.
I would suggest you reduce the flow rate as low as you can get it and still keep the roots wet. You do not want them to dry out. You dont have any medium to hold a thin layer of water like hydroton does.
You could certainly try an ON/OFF cycle, but I would keep a close eye on the roots for signs of drying out. During the OFF time, the water layer will be at its thinnest, so you should see some improvement in O2 transfer. Just dont let them dry out or you will be going backwards.
