Sand and Gravel

Sand and gravel are both potentially very suitable growing media for hydroponic systems.  The above is 10mm "sharp" gravel, which is commonly used in aquarium filtration systems, and could be used more often for hydroponics.  Note the irregular sizes/shapes, and the sharp edges, as opposed to pea gravel which is much more rounded. 

Sand and gravel are so common that I hardly need to explain what they are.  Yet many hydroponics growers overlook these materials as valid growing media candidates.  That's a shame, because they both offer some compelling advantages.  Yet they won't work for everyone, in all circumstances.

Advantages and Disadvantages of Gravel

One extremely common growing medium is plain old gravel. While gravel comes in a range of sizes and shapes, from sharp to polished, the rule of thumb for hydroponics is to use 3/8 to 5/8 minus, which means particle size will pass through a 3/8" or 5/8" screen.  So that size is the maximum particle size.  Additionally, most gravel users will wash their gravel before initial use, since some gravel comes with a high percentage of "fines", ie dust and dirt which would only serve to clog up filters.  I should point out that gravel often is also designated as either "sharp" or "pea" gravel, to differentiate between jagged edges and rounded/weathered edges.  Very generally speaking, sharp gravel is preferred because all those edges provide more surface area.  Surface area is important because that's what holds minute quantities of moisture in between irrigation cycles. The sharp edges also help keep the gravel mass from shifting, and gives the roots a better anchor than smooth polished pebbles would provide.  However, pea gravel will work if that's the only type available.  You might need to cycle your nutrient solution more frequently to compensate. 

Other advantages include gravel's relatively low cost and easy availability. Furthermore, it can be reused indefinitely. All the grower needs to do after each crop is to let the material dry out, then spread out in a large area, wash down to remove dead roots and other debris, rinse thoroughly, then put back into the grow beds. Few growing media offer such longevity particularly at such a low price. 

Yet gravel has its drawbacks. First, it is amazingly heavy. A small 2 quart bag of gravel weighs nearly 50 pounds. Any planting beds using gravel must be built to hold all that weight, particularly if they are to be raised above ground level for easy access. 

Secondly, gravel can change the pH of the water moving through the system. Both hydroponics and aquaponics rely on fairly narrow pH ranges for optimum nutrient availability. Gravel can change that pH depending on the parent rock material. Some types of rock will chemically react with the water enough to raise the pH levels far higher than they should be. Sometimes, that swing cannot be corrected even with acidifiers. If your water source is also alkaline, that single problem can virtually guarantee your pH would never be where it should be. Or it will cost a lot to fix it. On the other hand, if your water source happens to be slightly acidic, that gravel can provide built-in correction. So it pays to know your water's natural pH and then work from that basis as you select your growing media. And finally, gravel has no water holding ability. If you are using drip irrigation that may not be a problem. But if you want to use flood and drain, you may want to provide for backup irrigation or power supplies to make sure your plants are never left high and dry in case of a power outage, or plumbing blockage.

Advantages and Disadvantages of Sand

All the comments made about gravel also apply to sand, with one exception. The sand particles are already jagged enough that there is really no concern about sufficient surface area or anchoring characteristics. It is even heavier than gravel, so planting beds must be robust to support all that weight even if at ground level. It does not have any better water retention characteristics compared to gravel, but it does have better wicking abilities. So if a reservoir was built at the bottom of each planting bed, that water could wick up into the root zone more easily with sand growing media, as opposed to gravel growing media. That's not enough to supply your plants for long, but it can provide a buffer during power outages or other irrigation interruptions.

The major disadvantages to sand are the weight, and the extremely small void size between the sand particles.  Many growers who have experimented with both sand and gravel eventually end up with gravel because sand is just a little too heavy and hard to irrigate. Another related disadvantage is that filters have to be smaller than usual to ensure the sand doesn't run out through the irrigation cycle drain.  That means the filters have to be checked and cleared more often, usually by hand.

One instance where sand really shines is when folks have access to relatively large gravel types and need "filler" which can be rinsed repeatedly as part of a normal irrigation cycle.  In that sense, sand and gravel can be a wonderful combination.

As we saw with gravel, sometimes sand becomes the growing medium of choice simply because it is cheap and widely available.  Those characteristics can overcome a lot of other disadvantages in some circumstances.  If you have access to cheap or free sand, it's certainly worth experimenting a bit to find out if it might work well for you.

Additional Resources for using Sand and Gravel in Hydroponic Systems

Most growing media have been extensively researched, and multiple retailers and educational institutions have provided documentation about how well any given material worked out in hydroponic usage.  Sand and gravel have not enjoyed such attention.  Perhaps because they are both so common, they have received little such attention.  However, a few reports do exist which get into the details of using sand and gravel as hydroponic growing media.  One such report comes from the University of Hawaii, which describes the details of using sand and gravel in hydroponic growing systems for various crops.  Another from Colorado State University describes using sand and gravel in growing beds and/or biofilters for hydroponic production.

On a related note, sand and gravel are much more common in aquaponic systems.  For those who are not familiar with aquaponics, it is a combination of hydroponic growing systems and aquaculture systems.  Essentially, the fish providing the nutrients and the growing beds filter the water.  While the pro's and con's of aquaponics are well beyond the scope of this website, there are enough similarities between the two systems to consider aquaponic research results.  Given that, multiple reports exist for using either or both sand and gravel in small scale aquaponic systems.  One such PDF from the Southern Regional Aquaculture Center describes using sand and gravel growbeds and filters for a large home scale or small commercial scale aquaponics system.  A Google search of "aquaponic gravel PDF" will yield many more such reports.  While the filtration function is not applicable to hydroponic systems, the movement of nutrients through sand and/or gravel is very applicable to hydroponic systems and can provide important considerations.

Where to Find Sand And Gravel

While most hydroponic supplies will come from specialized retailers, either locally or online, sand and gravel can both be sourced from a wide variety of more general retailers. Home improvement stores and hardware stores can provide small amounts of both products, typically in either 25lb or 50lb bags. Larger bulk amounts of either product are also often locally available, but you may need to go a little farther afield. Landscaping suppliers, concrete contractors, topsoil suppliers and even local or regional gravel pits can provide either material by the container or truckload. Many of these businesses also offer a delivery service for larger quantities.  One additional potential source for small quantities would be pet supply stores which sell aquarium supplies.  Both sand and gravel are used as aquarium filtration and "landscaping" purposes.  If you need quantities beyond the small bags available at pet stores, yet smaller than what gravel pits would offer, discuss your needs with the pet store or home improvement outlet owner or manager and see if they can special order the quantities you need, possibly at a discount.