Should top layer of expanded shale be damp?

[bcp]

Hello,

This is my first post...  Yesterday I completed the pluming and I'm finally ready to start cycling my first system.  I'm using expanded shale in two 50 gallon rubber made stock tanks with a 100 gallon stock tank for the fish.  I'm using expanded shale for the grow medium.

I know I should set the siphons to leave about an inch of dry medium.  However after running through the night the top layer of shale is visibly damp.  I don't think the water is flowing up that high.

Does expanded shale wick water all the way to the top?  Is it OK to have the top layer damp or should I lower the siphons?

Thank You

[Chuck Goecke]

From my experience, expanded shale does wick water up pretty well, just as Hydroton does.  Both have tons of pores that are highly interconnected, and thus provide great capillarity plus tons of great active surface area, connected to the outside world.(like bioballs)  From what I've seen of Stalite, it doesn't wick as well, and is more like the Lava(scoria) pellets, which are basically foamed rock, where lots of the pores and vesicles not connected to the outside world.  This is probably why Stalite only absorbs about 6-10 percent water, whereas expanded shale can absorb 20-40 percent water of its 60 percent total porosity.  The 20-40 percent of it's pore space that retains air is cool too, because that keeps the roots aerated during the flood cycles, or if it is used out in beds in heavy clay soil, it help aerate the heavy soil, a property that Texas A&M has have great results with in our heavy Texas soils.

[Sterling Peyton Jr]

I hear people talking about 2 inches from the top now. So yes, cut your siphons.

[RupertofOZ]

I don't know where this 2" suggestion has come from....

 

It's always been suggested to flood to within 1" of the surface in the past.... particularly during summer, or for those in hot climates..

 

And partucularly if you're intending to direct sow seed...

Obviously some media wicks better than others... and generally the surface shouldn't be too damp... so flood height might need to be altered according to media...

[RupertofOZ]

In the same article Sylvia states...in relation to "flood & drain" systems... (although she illustrates an "ebb & flow" system)

so that the top one inch of the grow bed will be left dry

 

Probably depends a little on the media, and capillary action... and certainly relates to temperature...

But the only real reaquirement is that the level is below the surface of the media... to discourage and algae, or moss growth...

Personally, I think 1" is sufficient... and it was originally suggested as such...

Don't know why Murray would have recommended 2"... especially in a warm/hot climate such as Australia...

[bcp]

Thank you all for your advice.  I cut new stand pipes 1" shorter and put them in last night.  So far the top hasn't dried up.  I'll give it another day and see.  I can cut new pipes 2" shorter if that doesn't do it but that may be too short for the siphons to work.  I can play around with it during cycling.  With the 1" shorter pipes the siphons kick in a lot more so that's good.

[Sterling Peyton Jr]

RupertofOZ -  Your logic seems backwards to me.  In a hot zone, I would reduce my flood height to reduce evaporation.  The plants can go lower for water.  You do want to maximize your media surfaces to allow more bacteria area to convert your ammonia and nitrite.  

bcp - I hope you are understanding us.  We are talking about an 11 inch stand pipe in the siphon vs an 10 inch.  You seem to be talking about 3 heights.

[RupertofOZ]

Sterling Peyton Jr said:

RupertofOZ -  Your logic seems backwards to me.  In a hot zone, I would reduce my flood height to reduce evaporation. 

 

The plants can go lower for water.  You do want to maximize your media surfaces to allow more bacteria area to convert your ammonia and nitrite.

 

Firstly, the vast majority of any water lose in a planted aquaponics system... is via transpiration from plant leaves...

 

Not from direct evaporation....

 

But evaporation from the grow bed surface can certainly be increased in an unplanted, or newly planted grow bed...

 

Certainly once a seed, or seedling has developed... the tap root can extend downwards into any water zone...

 

But in the early stage of growth... and with many plants.... the early root development is very close to the surface of the media... and an "unwatered" hot surface zone can easily burn off a young seedling, or stop germination of sown seed...

 

This is a reason for actually employing a "constant flood" regime for newly transplanted seedlings... or direct sown seed...

 

Secondly... the nitrifying bacteria populate the myriad of "holes" on a media surface... within the thin film of water held by surface tension....

 

Obviously, for them to convert the ammonia, and subsequent nitrites...in the water... to nitrates within the water, for our plants to feed upon... they need to be in contact with the water...

 

Effectively... any "unwatered"... or "unwicked" dry zone... is essentially unoccupied by the nitrifying bacteria....

 

The raised temperature of the "unwatered"/"unwicked" media in itself.. would probably inhibit, or kill any bacterial colony in itself...

 

Thirdly... plants, and the bacteria... derive the vast majority of their oxygen intake... through the root systems, via water osmotic transfer...

 

Fourthly... the more the media heats, and transfers heat to the water... the less dissolved oxygen is available to both fish and plant processess....

 

The only reason we don't flood to the actual surface of the media itself... is that the UV exposure of the nutrient rich water... would develope algae, or moss... which then coats the surface of the media bed....

 

And limits the draw of oxygen through the grow bed... to the detrement of both fish and plants...

 

And the algae or moss actually draw oxygen from the system itself for their own processes....

As you say...

 

You do want to maximize your media surfaces to allow more bacteria area to convert your ammonia and nitrite. 

 

 

There is just no benefit in reducing your filtration capacity...and/or oxygenation potential... by not flooding to the highest level possible within the media.... as long as it doesn't invoke algael/moss growth...

 

[Rob Nash]

@ Sterling, i think Rupert was only referring to "getting water to the smaller plants" not so much about the effect on temp or evaporation. ...i hear what your saying, it would seam best to avoid the top layer of the media in the hot months to keep from heating up the water.

...i have noticed the water getting hot if it makes contact with the very top layer of gravel, but gravel/media does not transfer heat as much as one may think. ...i would be more concerned about the temp of the water than evaporation. in my experience, within one inch is ok

I'm starting to build my media beds at 16" ...that gives room for 14" of media and thus 12"-13" of wet/root zone...the 12" sides only allow me to have about 9"-10"  of media and 6-8" of wet.

Cheers,

PS - leaving today to go to Denver, my wife and I are going fly fishing before the conference. see y'all there!

[Rob Torcellini]

All of my beds are filled with expanded shale and they flood to about 1 inch of the surface.  The shale stays damp all the time and has not been a problem and have not had any algae growth.  I did have one set down to have the water level about 3 inches below the surface and the shale was a bit drier, but not by much....it just wicks really well!

[Chuck Goecke]

I wonder how many have actually had a problem with algae and moss growth?  I only recall seeing algae and moss being a problem in one of our(my former company's) propagation greenhouse and then only on the large stock plants that were used for cuttings for propagation.  They were in potting soil, and were just old old plants, multiple years old.  We got algae, moss or liverworts growing on various plants, as well as fern babies and a couple of higher plant weeds that always vexed us, like oxalis and a ferny looking Pilea.  The bigger weed plants we just had to pull, but the algae and moss just needed the soil surface to be raked and broken up. 
I would suspect that in a grow bed, where you are re-cropping every few to maybe 6 months, there would not be time for the kind of algae/moss crust to form.  If you had red wiggler worms in the bed, they would eat the moss/algae.  If you did start seeing some green on the surface, simply raking and disturbing the surface, between your plants would solve the problem.  
I'm all for getting maximum filtration usage out of the medium, which would mean flooding as high as possible, as long as this doesn't adversely effect your crop.  If you are having problems with damping off and other stem rot fungi, like Phytophora, that is probably a combination problem with too much humidity and too much/frequent water.  If it is very humid, decrease the flood frequency, and level, if possible.

[RupertofOZ]

Agreed Chuck, in reality very few people probably encounter the problem these days.... as it's become accepted to maintain the flood level 1" .. at least... below the media...

 

The most often occassions when it may be seen... is when people splash, or dump the water into the grow bed above the level of the media...

 

Rather than as a result of actually flooding too high...

 

Or because of root ball development underneath a grow bed inlet... or in root bound beds due to long term heavy root system plants... i.e tomatoes....and thus subsequent water "pooling"...

 

[bcp]

Thank you all, I put back in the original stand pipes then dug a small hole and watched it fill.  The water stops about an inch and a half from the top of the shale.  But the top of the expanded shale looks wet.  I think Rob Torcellini is correct that expandable shale wicks very well.  I'll just leave it alone for now and see if I have any problems with algae down the road.