Sand and gravel is not porous, so it doesn't retain moisture real well in general. Depending on the grain/grit size, it will drain well (however that depends on how you design the system also). The finer the grain, the slower it will drain, also less air will be able to get to the roots. How often to run cycles will like always, depend on a lot of variables, so basically that's a trial and error type thing. It's my understanding that it works well in drip systems. In fact sand is the growing medium of choice at Epcot center for drip systems (I don't know if they mix it with gravel).

It can be done in flood and drain systems, but not what I would choose unless I was bent on a flood and drain system for root vegetables. Besides the liquefaction problem that water saturated sand presents (where plant stability will be a problem), it will be much harder to filter the sand from getting in the system (pumps, reservoir, tubes etc.). That will be much easier to control in a drip system.

I just happen to have 50 pounds of 16 grit silica sand soaking in some bleach water to sanitize it for a hydro system right now. It fills the 5 gallon bucket up to 4 inches below the top lip of the bucket. I would guess this is about 4 gallons worth of sand. 105 divided by 4= 25.5 (50 pound bags). Or 1312.5 pounds of sand. I don't know what the difference would be if you mixed gravel with it.

I bought it a couple weeks ago for a system I plan to use for growing small root veggies like radishes and green onions. It's my understanding that root veggies grow better in growing mediums that has a more soil like texture like the sand. They tend to be odd shaped in lighter mediums like coco coir perlite etc. and/or larger more coarse mediums like grow rocks. I also plan to mix it with vermiculite for water retention. The combination of the two should give a good balance between weight, texture, and moisture retention.

The 16 grit sand was the largest particle size I found, and I chose it because it will allow better aeration of the roots than the smaller particle size (the higher the number the smaller the particle size). Silica sand is not exactly the cheapest (about twice as much as play sand), but it looks like what they use at Epcot. Also they say it's hazardous to breath the dust, but that wont be a problem because the process of sanitizing it will wash the dust out, and the sand will be wet in the system anyway (so it wont be airborne).

Play sand should work fine also, however I don't think the grade the particle size like they do for silica sand (at least not what I saw). My project is on hold though until I can afford some more vermiculite (I already had everything else). The container I have planed for the system holds as much as 9 gallons of growing medium, and will be a drip system. I figure I need about 2 gallons worth of vermiculite (2 to 1 ratio sand and vermiculite). That should fill the container 2 thirds the way (about 4 inches deep), that's probably just enough for the plants I plan (particularly the radishes).

Radishes and green onions are not usually very expensive at the stores, but I would really like a steady supply of fresh green onions (that we tend to use a lot of when we have then in the frig), and I like eating radishes by themselves with a little salt on them (as well as salads). I will also probably be able to grow some other things in the same space like chives. Over all it should be a fairly economical system, however in the future I might opt for the regular play sand instead of the silica sand, just because of the price difference.

I plan to be able to harvest them and replant in the same container. However I will probably need to add beneficial microbes/fungi to the nutrient solution to deal with decaying roots from harvested plants that were left in the growing medium once harvested.

Thanks Homehydro;
Having grown up on the Atlantic seaboard, it's easy to comprehend the liquefaction concept. The sand is relocated with every tide. I LOL when the news media uses the phrase "A line in the sand." When a nor'easter shows up you can run out of beachfront real quick! Flooding such a system would have to be done at a snails pace to avoid relocating the sand therefore the nutrient solution would more than likely return to the reservoir faster than the grow container could fill, unless a valve was utilized to control the flow back to the reservoir. Way too complicated and too much to go wrong. Back to the drip method.

What about Perlite? Now I'm thinking along the lines of 50% Gravel, 25% sand, 15% Perlite and 10% Vermiculite. I realize this will add a bit to the cost, but it may be worth it to make compromises in the financial department.

My guess is that the beneficial microbes/fungi don't actually clean up the decaying roots, but rather attach themselves to live roots, Protecting the plants from the harmful bacteria/fungi propagated by the decaying roots. Do I have that correct? Most Mycorrhizae/Trichoderma products don't state exactly which strains they contain, But Trichoderma Harzianum (no I can't pronounce that) keeps popping up in my soil borne disease searches. Are there other beneficial strains known to specifically protect from Fusarium/Verticulum? I'm just tired of having my entire tomato crop looking like it's been thoroughly vandalized with battery acid shortly after mid season. I've really had it with this stinking organic composting nonsense!
Thanks again,
~Ken~

As far as I know gravel is a fairly generic term, and comes in lots of different sizes. Anywhere from say river rock, to large grit sand (as in the 16 grit silica sand I got). Bottom line if using river rock sized gravel (or anything similar) the smaller sand particles will easily take up the space between the larger gravel/rock particles. And when you refer to gravel it's hard for me to picture what your referring to in that regard. The only benefit I can see in using gravel is the larger particle size allowing more air between the particles. That aids in oxygenation of the roots, and drainage. However if the smaller sand takes up that space I'm not sure I see a benefit in mixing the two. So I guess it comes down to just how much difference there is between the two different sizes.

Perlite is another good medium to mix with sand/gravel mixtures. Perlite is more porous than vermiculite, and will help aerate the mixture better than vermiculite. Perlite will also help retain moisture in the sand/gravel mix. So I cant see any problems using it. I may even decide to use some also. Although Perlite is very light, and I don't think I will use a lot of it in the mix for my root plants. Simply because I want to maintain weight to the mix (as in traditional soil), and don't want it to become to light. That's the main reason root veggies become deformed from what I understand (but they are still definitely editable, just the shape may be off). That wouldn't be a concern for non-root veggies. I had planed to only add vermiculite because at the time I had plenty on hand that I bought last spring, however most of that is used now and I don't have enough anymore.

"My guess is that the beneficial microbes/fungi don't actually clean up the decaying roots, but rather attach themselves to live roots, Protecting the plants from the harmful bacteria/fungi propagated by the decaying roots. Do I have that correct?"

For the most part that's exactly correct. The beneficial microbes aren't actually there to devourer the dead/dying roots. Although there is one product that claims to do just that with enzymes (CANNAZYM ), but I haven't had time to look for creditable sources of infromation on it, and find out if it actually does what it claims to.

But dead/dying roots are a feeding source for pathogens. The beneficial microbes will help keep their numbers down and from breading out of controls by feeding on the pathogens. But both pathogens and beneficial microbes will not only live on the roots, but within the growing medium itself, as well as in the water.

By adding the beneficial microbes/fungi/acids to the solution, the entire growing medium's surface area becomes essentially a bio-filter that will continue to breed the beneficial's, as long as they have a food source (the pathogens) and good living conditions (namely the right water temp). Water temp is also a factor that affects how fast pathogens breed and multiply.

I am not well versed in specific pathogen and soil born diseases, but I have seen a list of the different strains and what they protect against. Though I'm not sure if I have it saved to my computer somewhere, bookmarked it, or even if it covers your question. But it will take me a little while to see if I can find it again. I have a lot of articles saved and would be happy to post them if it may help, but you seem to know what your looking for. I wouldn't be able to tell what pathogen caused what. It is my understanding that a high quality beneficial microbe/fungi product will have a wide variety of strains in it. So for the most part I just plan to make sure I use high quality products for that.

High Quality Trichoderma products
Liquid or granular formulation
Mixture of Trichoderma species
High CFU or spore count
Long shelf life"-18 months minimum
Reputable manufacturer
Active over wide temperature range
Organic certification

Low Quality Trichoderma products
Refrigerated product
Single species product
Low CFU or spore count
Short shelf life
Questionable source
Active over narrow temperature range
Not organically certified

I see what you're saying and I agree. It makes no sense to mix fine sand with coarse gravel. I was thinking along the lines of the coarsest affordable sand I can find and the smallest affordable gravel. The gravel I have in mind would be similar to the stuff they mix with Tar around here to resurface roads. I call it gray Trap Rock because as a youngster I remember watching barges haul huge boulders of it down the Hudson River that said "New York Trap Rock"
I don't know how they measure gravel but The stuff I've seen averages about 3/8 Inch in size and varied in shape, but predominately shaped like twisted cubes with no round edges. It's just slightly larger than aquarium gravel. I've also been thinking why not just use the gravel and trickle the solution 24-7 through several drip rails? In concept it would be a three dimensional NFT system. The theory being that the medium need not retain moisture so long as the solution is being delivered evenly and constantly. An added bonus is the lack of sand ending up in the reservoir, pump, lines, etc.
What do you think?
~Ken~

Size wise, I'm thinking aquarium gravel size should be fine. I don't have any to mix with the sand to see how it feels mixed together though. What I do with my drip systems is put rock at the bottom of the 5 gallon buckets. say about 1/4 to 1/3 of the bucket, then fill the top of it with the growing medium (coco chips). This does three things, one it takes up space so I don't need to use as much growing medium. Second it provides for good drainage at the bottom of the buckets. Third it adds weight to the buckets so it wont become top heavy or have any chance of blowing over in the high winds we get from time to time.

I don't know how well it works with sand as the growing medium, but I might try using a inch or so layer of fish tank size gravel at the bottom of the growing container for my root veggie system to take up space and add to the overall depth, as well as possibly aid in drainage if too much of the sand doesn't filter down.

The way I have planed to drain the system is, drill a bunch of small holes (about 1/4 inch) at the bottom of one container. Then lay down a layer of cloth like material. What I actually have is that white cloth they sell at Christmas time to put on the mantel or no table tops so it looks like it is snow covered (I got it on sale in Jan for 10 cents). But a few layers of cheesecloth should be just fine. On top of that I plan to put down a layer of furnaces filter screening material. I can get a cheep one at the hardware store or Wal-Mart for about $2. On top of that I will place the growing medium (sand). The two different type of screening materials should keep 99% of the sand in the growing container, but still allow the water to easily drain. With plenty of holes, I wont worry about clogging, Even if the roots grow down through some of the holes.

In a second identical container, I will have a single through hole mounted at the bottom. A tube connected to the through hole will drain the water back to the reservoir. Then I just set the first container inside the second one. Because the lip of the through hole is a little higher than the bottom of the second container, that should trap any sand that somehow makes it through the screening material. But if all that fails to catch all the sand from getting in the reservoir with the pump. I'll create a trap just like under the sink (that U shaped section of the pluming). Because sand is heavier than water it will sink to the bottom of the trap, and the water will overflow out the top into the reservoir. Then I would just need to empty the trap once a week or so.

I have seen where people have used gravel and just ran the dippers 24/7. In fact a lot of aquaponics setups are like that, so it should work fine provided you can evenly wet the gravel. But personally I think the plants would have better root structure if the water is not flowing 24/7. The reason being is because they would basically be submerged in flowing water, and I feel they would get more air to the roots if they don't have water covering them all the time. By allowing more air to the roots, that will allow them to grow more of those small fine roots that take in a lot more of both water, and nutrients then expand the root structure as they get bigger.