Sand

[Harold Sukhbir]

A while back someone suggested sand as media being superior to gravel.After some searching i came across this article on lettuce grown in sand vs gravel with very interesting results. Traditionally i think sand was used early on in AP but was found to have problems associated with it's use (water retention,anaerobic problems,associated hardware problems etc.) Does anyone use this media and if so can you share your experience with it?

Here's the article...

mediaandfiltrationexperiment.pdf

[Dan Brown]

Great piece of reference material!  I have no experience with sand as a medium, and nothing to add to this discussion, but it seems like we need abundant documentation that demonstrates the differences which manifest in alternate growing mediums.  This study is intriguing, if for nothing else that optimizing planting arrangements to take advantage of different nutrients available through different techniques.

[Sylvia Bernstein]

Yes, very interesting article, Harold.  Thanks for this.  If I get this right, they were floating the hydro units on a bamboo raft on top of a catfish pond and used 2 filtration regimes - unfiltered and partially filterd (I'm doing this partly for my own purposes so I can envision it in my head).  They then grew lettuce in 3 mediums - styrofoam (pearlite?), sand and gravel and irrigated 2x a day.  My guess is that is why the sand did better.  

 

The problem with sand, as I understand it, is that it gets clogged up too quickly when the media is being used as the filter in a flood and drain system.  And that is when we are constantly irrigating.  Even in a traditional hydroponics system you will tend to fertigate (I love that word) every 4 hours during the day.  Because the experimental protocol calls for such an infrequent cycle the clogging issue would be lessened...and replaced by an issue of not getting enough nutrients to the plants!  So I'm not surprised that the one with the highest surface area would win because it can hold on to the most nutrients during those two floodings.  Does that make sense?

 

The advantage to gravel as a media is that it also works well as a filter.

 

But that's just my wild theory...

[TCLynx]

I've known of some people who tried sand in some barrels once.  And I did try sand in my top tank on my barrel poncs for a little while.

 

I agree with what Sylvia said though I haven't read through that whole PDF just a few bits.  The sand is just too dense to really work well a a flood and drain media the way we use gravel.  It will hold too much moisture and clog too easily while not allowing enough air down into the beds.  While these things were probably an asset when only irrigating a small number of times per day, it won't work to replace gravel beds with sand unless you provide other means of filtration and then only water the plants a few times a day.  (Which might not keep the water quality very good for the fish.)

 

However, you now have me thinking I might try sand with some wicks to start seeds or something.

[RupertofOZ]

Yep, the devil is in the detail..

 

Water was pre-filtered.... sand bed was watered twice per day for 45 mins.... a total of 90L per irrigation... and the yield, as opposed to unfiltered water... increased by 52%...

 

The same irrigation method saw a 63% increase in yeild... in gravel....

 

For all the reasons outlined by TCL, and Sylvia.... sand just wont work in a continuously flood & drained aquaponics system...

 

Particularly one fitted with a siphon... which would just suck sand throughout the system... pumps and grow beds....

 

Even in the early days of hydroponics... i.e The "bengal" system... gravel/crushed brick/stone... was utilised over sand...

 

Wastewater treatment plants, swimming pools, and even some RAS fillters...utilise sand for filtration... because it traps everything... but for the same reason... they require regular and extensive back flush treatments... and periodically the sand needs to be replaced...

[RupertofOZ]

What I found interesting in the paper cited... was the fact that yeilds increased in the filtered water... with a corresponding decrease in Phosphate levels due to the filtration...

 

That doesn't surprise me at all... particularly with lettuce... which just doesn't need a large phosphorus provision... basically just nitrogen and water....

 

IMO... the same applies to most plants.... and the case for high phosphorus NPK fertilisers utilised in old style agriculture... is more to do with soil pH and phosphorus retention/leaching....

 

Commonly, soil based agricutlture apllies both lime and superphosphate.... chacing it's own tail so to speak...

 

And I say this because most pond based aquaculture applies lime to "spelled" ponds before refilling.... to release the phosphorus laid down by wastes during the previous grow out period... phosphorous that is otherwise "bound" or "retained"...

 

And the study is a case in point... a pond based aquaculture system... with (no doubt) lime applied... and reflected in the pH... principally for pond base algal bloom management techniques...

 

I truely belive that phosphorous , while traditionally considered a "macro" element... is actually more a "micro" element... for most/many plants...

 

And,as is the case with most AP systems... in acidic pH conditions... is freely available for uptake in the "micro" amounts needed by plants...

 

Most pellet feeds contain all the phosphorous needed in an AP system IMO...

and a balanced flood & drain media, well oxygenated system, with worms.... is all that's needed...

[Sylvia Bernstein]

Fascinating, Rupert.  I've never thought about it this way, but have reached the same conclusion just with my own growing.  I just had the second article in a 2-part series I wrote for Urban Garden magazine show up  online and got a comment yesterday "would like to see a continuation of this series to include possible nutrient deficiencies to supplement the nitrates and aquatic poop nutrients."  I basically replied that that would be a very short follow-up article because there are no systemic nutrient deficiencies in aquaponics.  Sometime people need to supplement iron, but I'm pretty convinced that is more of a pH issue than an actual deficiency.  The hydro world (which is the Urban Garden mag audience) has a very hard time wrapping their minds around this...and I can understand why.  It is pretty amazing.

[Sylvia Bernstein]

Rupert, what do you use to buffer up your pH?

[Harold Sukhbir]

"For all the reasons outlined by TCL, and Sylvia.... sand just wont work in a continuously flood & drained aquaponics system..."

The comparison in the control of sand to gravel even with overstocking fish( i believe to increase nutrient levels due to anticipated reduction in irrigation cycles) was unfair when we consider standard AP.In Ap we get around low nutrients by regular cycling creating an increase in bioavailability to our plants. If the control had been watered say 12 times/day the outcome would have shown much different results.Even the growth rate of the plants are not suitable for backyard AP more so for commercial application.What about results over a period of one year for the standard control? I'm pretty sure issues like root rot and anaerobic problems would have surfaced. I agree with you all that gravel as media is superior to sand.

[Robert Robertson]

I too thought of sand as a media because I have it in such abundance.  What I'm fixing to write has no connection with aquaponics other than the concept of what would I suspect happen.

 

I used to take care of a 150,000 gallon public swimming pool built back in the 50's.  Believe it or not they had 4 5x5 pits for filtration.  The pits had about a foot of gravel on the bottom and 2 feet of sharp sand.  Water came in the top of the pit and the pump sucked from the bottom (hence the need for the gravel)  Water turnover rate was a little more than three times per day.

 

Believe it or not the pits were very very efficient at their job.  They had to be backflushed twice per week or you would notice a change both in flowrate and water clarity..  Here is the part that may be of interest to you.  At the end of a few days each pit would have soft spots in the sand where water would just simply channel thru, bypassing the rest of the filter.   Outside of these channels the sand was packed -HARD.  When backflushing, You literally had to get in and walk around to get the sand mixed up.  Actually that was kind of gross.

 

While the flowrates in an aquaponics system may not be that much,  I still believe that in short orde,r channeling would occur in the sand.  That seems like it would kind of defeat the purpose in as far as getting nutrients to the plants and filtering.  I don't know what all the anaerobic stuff means-sounds like stinky dirt to me, but physically I have seen this happen to a sand system.

 

Hope it gives you something to think about

Robert

[TCLynx]

Yes Robert, I think your description of the sand filters does illustrate the problems with trying to use sand in a grow bed for aquaponics quite well.

 

I did once try sand in one small grow bed for aquaponics, the flow through it just didn't remain good enough.  Sand filters just require too much backflushing/cleaning.

 

Now if there is separate filtration going on for a system, I suppose some sand beds that only got irrigated once or twice a day might provide a place to use some sand but it doesn't really seem all that efficient for a normal set up.

[Rick Op]

Water retention due to capillary effect goes up as particle size goes down.

Sand is really small particles of silica gravel.

It will therefore retain water "a whole lot more" than pea gravel - functionally it will hold water until it evaporates or it is displaced (flushed) from one direction or another. Gravity will NOT be able to break the capillary hold and drag the water/nutes/icky-bits down out of the sand into any "drainage layer" you place beneath it. Ain't gonna happen no matter how long you wait.

It follows logically that...

Even if channeling were not a factor,

Even if you pre-filtered solids out to prevent Fish Poo Pavement,

Even if you installed some sand worms (aren't these fictional anyway?) to keep it stirred and un-anaerobic,*

Even then your plants would have constant wet feet... so bananas and mint and watercress would be just fine with it. Others would need to be tried to see how they do. There might be other wet-tolerant plants. Would be a good thing to see if sand worked better for these than larger gravel.

 

Rick

 

* Sorry it's a double negative (and likely not even a real word). I'd have tried for a triple but it's still early in the day.

[Harold Sukhbir]

I agree Rick, it's no wonder the testing was done with lettuce alone, but what got me in the conclusion was that the author recommended sand for consideration as a hydroponic media due to his results. I think there will always be a missing link between science and practical AP experience,unless you get an AP practitioner who is also a scientist.

Rick Op said:

Water retention due to capillary effect goes up as particle size goes down.

Sand is really small particles of silica gravel.

It will therefore retain water "a whole lot more" than pea gravel - functionally it will hold water until it evaporates or it is displaced (flushed) from one direction or another. Gravity will NOT be able to break the capillary hold and drag the water/nutes/icky-bits down out of the sand into any "drainage layer" you place beneath it. Ain't gonna happen no matter how long you wait.

It follows logically that...

Even if channeling were not a factor,

Even if you pre-filtered solids out to prevent Fish Poo Pavement,

Even if you installed some sand worms (aren't these fictional anyway?) to keep it stirred and un-anaerobic,*

Even then your plants would have constant wet feet... so bananas and mint and watercress would be just fine with it. Others would need to be tried to see how they do. There might be other wet-tolerant plants. Would be a good thing to see if sand worked better for these than larger gravel.

 

Rick

 

* Sorry it's a double negative (and likely not even a real word). I'd have tried for a triple but it's still early in the day.