Water Drain Depth

[Jagungal]

So .. I have been playing with my system the last few nights in the hour or so I get before dark and Mossies take over.

I made a bigger surround for the Siphon but found that it makes little difference whether I use it or the smaller one - or there is media or not in the bed. At the moment it is draining well but leaves about 50 mm of water at the bottom of the bed before kicking off.

The question would be how much water should be left in the bed on the drain ? Is 50 mm about right or am I being anal worrying about trying to refine designs to leave less water in the grow bed ?

[arachdog]

Gary will tell you it doesn't matter. Maybe I'm a bit anal myself but the fact is any roots that are in the permanently flooded zone will probably only function half as well (if your lucky) as the roots with regular access to oxygen. Only you can decide if that's significant enough to worry about. If the half efficiency estimate is true (I don't think its far off from memory but probably worth finding out for yourself) and your crop has an even vertical root distribution then a 50mm permanently flooded zone in a 300m bed would mean the bed would produce at only 92% capacity. But possibly a lot of crops don't have even vertical root distribution, which would change that.

[Ravnis]

some points to consider:

In raft systems the roots are flooded 100% of the time.
A water reservoir in the bed is added protection to keep the roots from drying out should a flow problem occur( pipe breaking or coming apart, pump failure etc.)
An anaerobic zone is useful for converting some elements into plant usable forms. Iron is one example.

I personally would not worry about it.

[Jagungal]

Thanks for the replies ....

I did some more experimentation last night, increasing the length of the drain pipe under the grow bed by about 10 cm more on the pipe. Now the bed drains to 20 cm or less depth which is about the height of the inlets. This length of drain down is what it will have when operational and what I had before.

I was hoping that a bit of water might be good left in the bed, I do have a small drain hole that continuously lets water out so if there is some left in there, it will drain in time.

Time to start cycling .....

[GaryD]

Hi Jagungal,

Gary will tell you it doesn't matter. Maybe I'm a bit anal myself but the fact is any roots that are in the permanently flooded zone will probably only function half as well (if your lucky) as the roots with regular access to oxygen.

Arachdog is right......I will say that it doesn't matter.....for the reasons articulated by Ravnis. In my last system (and probably in my proposed ones), I ran sub-surface continuous flow so I always had water in the beds.

While I like auto-syphons, I got rid of them in the interests of simplicity. Continuous flow watering works every bit as well as flood and drain.

Gary

[arachdog]

I'm not a big fan of raft systems. It was one of the first hydroponics methods, but it rapidly fell into dis-use as it was displaced with more effective methods. It's little better than soil in term of efficiency because the roots are in such a low oxygen enviroment (100% saturation in water is nothing compared to atmospheric exposure).

[Jagungal]

Sounds like nothing much to worry about whether the drain works great or not.

At my estimation, there is still going to be about 200 - 250 mm of growing depth anyway and I suspect that is plenty especially if some people are running continuous flow systems.

Assuming that DWC is the same as flowing raft, there does seem to be commercial systems still using it. I had the impression that not all of the root was in the water and that only part of the media was in the water which allowed it to draw water and nutrients up for the plants use - but that you needed a reasonable amount of water movement to keep the nutrient flowing.

[GaryD]

Arachdog....

I'm not a big fan of raft systems. It was one of the first hydroponics methods, but it rapidly fell into dis-use as it was displaced with more effective methods. It's little better than soil in term of efficiency because the roots are in such a low oxygen enviroment (100% saturation in water is nothing compared to atmospheric exposure).

It's also the most successful commercial AP model to surface so far........and it has 25+ years of research underpinning it. The most quoted raft system (UVI) ensures adequate oxygenation of plant roots by installing air stones in their growing troughs (which are each about 30m long) at 1200mm intervals.

The raft system has been mirrored by many people in various countries.

The only other hydroponic method that comes close to it (in terms of frequency for commercial use) is NFT. This two main issues with NFT seem to be managing root zone temperature and keeping the system clean and free of blockages.

If I was to start a commercial system tomorrow, it would be a raft system.......and that would be the advice that I'd provide to anyone else, too.

Gary

[arachdog]

If thats the AP model that has the most research behind it Gary then I agree it probably is the least commercially risky at this point. But as the field develops, I'd be very surprised if it ended up on top. Those NFT issues you mentioned are certainly not insurmountable. The root zone temperature issue is an easy fix, don't make your growbeds black. Avoiding blockage are really about correct sizing of drains and beds, combined with regular maintenace.

[GaryD]

Hi,

If thats the AP model that has the most research behind it Gary then I agree it probably is the least commercially risky at this point. But as the field develops, I'd be very surprised if it ended up on top.

And what do you think might replace it?

The reason that it has the most research behind is because it was the system that worked. Dr James Rakocy tried grow beds 25 years ago and rejected them for commercial purposes.

In my view, (and aside from anything else), NFT is too fiddly when compared to raft.

Those NFT issues you mentioned are certainly not insurmountable. Avoiding blockage are really about correct sizing of drains and beds, combined with regular maintenace.

NFT has also been around for a very long time now. Dr Allen Cooper developed it in the mid 1960's if I recall correctly.

The root zone temperature issue is an easy fix, don't make your growbeds black.

Actually, most of the NFT troughs that I've encountered have been white. The root zone issue has more to do with the relatively small nutrient flows being exposed to high ambient temperatures. Cooling towers and other strategies have been employed to counteract the problem.

Raft is much simpler to put together and more stable because of the much greater volume of water.

Gary