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Should I use Lavarocks or Expanded Clay?

  Added by: snoofer  Last edited by: snoofer  Viewed: 485 times   Rated by 10 users: 9.70/10
Contributed by: snoofer
Thanks to: Aiptasia, the colonel, Snaps_Provolone, OzHornet, tdmaker, nuniabiz, Son-T, raygun, 10k
Images: Philcuisine and unknown

Gravel, sand, perlite, rockwool, oasis cubes, coco, expanded clay (?poprocks? or Hydroton/Groton/ L.E.C.A. and other names), and lava rocks are common choices for Hydroponic mediums.

Lava rock and poprocks are popular as they are inert, do not absorb water, provide good aeration, are Ph neutral, and can be re-used.

Which is better?
Both of these mediums have advantages and disadvantages, depending on the system and requirements. Lava rocks work better in some systems, poprocks in others. They are good for drip, nft, flood&drain and for filling mesh pots.

Both types should go into a system that irrigates frequently, as Hydroton and lava rocks are non-porous and have little water-holding capacity. Both types should be pre-washed and sterilized to remove dust & mold.


Thanks to: PhilcuisineLavarocks are a cheap and readily available medium. They are good for large bucket grows where stability is a necessity.

(Aiptasia)”… consists of feathered basalt which is pH neutral. Basalt can carry trace metals, and my big lava rocks rust from the trace metals…”

(Snaps_Provolone) “Lava rock is chunks of red, porous (VERY porous!) basaltic (volcanic) rock. I’ve purchased it both by the bucketful, and by the bag at any place that sells landscaping materials. It is much lighter than, say, peagravel, but still rather heavy.

Only rockwool (MUCH more $ than lava rock) has more air/nutrientfilm capacity. Lava rock has REAL good capillary action too. Once wetted (I use flood-drain/ebb flow), it holds VAST amounts of water, while affording roots WAY more space to devolop than rockwool EVER can.”

Lava rock is good for ebb-flow/flood-drain, or top-drip systems, but can also be used successfully in an airated standing solution.

I still prefer Hydroton though, it stays moist longer and lacks that sharpness that tends to be harmful to soft tissue..”

  • Heavy. Roots will become damaged if a plant shifts; heavier lava rocks will help stabilize a large plant, preventing it from leaning or shifting during growth.
  • Chemically inert and reusable. Does not absorb water.
  • Lava rocks ?lock? together to give a more solid and stabile medium to plant in. Large plants are less likely to shift.
  • Easy to find, usually inexpensive
  • Lava rock come in large, medium and pellet sizes.
  • Individual rocks can be hand-placed to anchor air stones, drip lines, stakes, etc. Stakes embedded into the rocks will be solid.
  • Lavarocks can take a beating and not fragment or crush easily


  • Lavarocks need to be pre-soaked to clean them of grit and stabilize the pH:
    (raygun) “if you use the lava rocks just make sure that you rinse well and soak them in ph balanced h2o for 3-5 days then rinse and use.”
  • Lava rocks are a pita to clean. They have irregular surfaces and micropores that roots and bacteria love to fill. 100% sterilization is never guaranteed. Some growers consider them one-grow disposable.

    (the colonel) “i find it easier to just buy new lavarock and rinse/clean it all than to reuse it: scrub each individual lava rock clean off all traces of root, and then disinfect and then rinse real well to get all the disinfectant chemical out.. but thas just me”

  • Lava rocks are heavy. This makes everything heavier, harder to haul/move, and more expensive to ship.
  • Lava rocks may contain traces of heavy metals, which may cause nutrient deficiencies and pH swings.
  • Irregular rock shapes provide uneven aeration and wetting in the rootzone. Roots will be less able to penetrate evenly throughout the medium

    (raygun) “I did not like using the clay by itself as it did not disperse the ater from my drip tube and I ended up with dry spots in my pots. The lava roxs are all shapes and sizes which help change the path of the falling water.”

  • More difficult to fill containers

    Expanded clay / Hydroton

    (Son-T) ?Hydroton: This Lightweight Expanded Clay Aggregate (L.I.C.A.) is manufactured exclusively at high-tech kilns in Germany and is used as a soil replacement in hydroponics. Composed of shale that is pelletized and fired, Hydroton holds water extremely well and transmits it effectively. The pellets are uniform in size and have an attractive, natural appearance. Hydroton is chemically inert, has neutral pH, is reusable, clean and odorless.?

    (nuniabiz) “clay balls at like 2800 degrees which expands all the little bubbles in the clay making the surface area something on the order of 100 times greater theres for holdin more o2 and wicking more H2o purrrrrfect”

    The high temperature sterilizes the Hydroton, but mold can still form in the bags due to condensation and breaks.

    (OzHornet) “marble-sized orange/brownish balls you can get from nurseries or hydro stores – often used in aeroponics; very cheap”

    Poprocks are the preferred medium for hydroponics.


  • Expanded clay is much easier to clean. The round balls of clay have a smooth surface coating that can be cleaned of roots and bacteria.
  • Chemically inert and reusable
  • Different sizes available
  • Lighter than lava rock
  • Poprocks are more uniform in size & shape. They can be poured into containers, and they fill containers evenly.
  • The round pebbles do not compress or touch. The uniform space between the pebbles provides even aeration and wetting throughout, allowing roots to fully grow into the medium. The clay surface repels water, but surface tension coats each clay ball in a thin coat of water ? perfect conditions for roots. Cracked rocks absorb water like a sponge.
  • The round pebbles have more surface area than the flatter lava rocks, proving more area for roots to cling to.


  • Can be more difficult to source
  • Poprocks are round and as such are less stable to plant in. Larger plants may shift if disturbed, damaging their root systems.
  • Poprocks both sink and float! Poprocks can clog drains and tend to go everywhere. Knock a pot over and you?ll be picking up pebbles for a long time
  • Poprocks need to be thoroughly pre-washed to remove the heavy clay dust that is caused by the balls rubbing together during transport. Clay dust will still come off them and may require a flush of the system upon startup. Heavy clay residue will settle to the bottoms of systems and may clog pump filters.
  • Poprocks will slowly break down, as they are exposed to acidic nutrient conditions.
  • Poprocks can break, shatter and become crushed with handling

    Rinsing tip for both lava and pop rocks:
    (tdmaker) ?Simply, poke holes in the bottom of the bag and cut open the top. With a water hose, run water through the top of the bag. The red powder will flow from the bag through the holes in the bottom. Oh, and do this outside. Otherwise, do smaller amounts indoors?

    Should I sterilize my used medium or encourage bacteria?

    (10k) As far as harboring beneficial bacteria, user ‘Jackerspackle’ wrote some extensive material on this several years ago.

    Basically what he said and I believe and practice, is that expanded clay mediums harbor them too. Just dont sterilize your rockage when washing them out and you’ll have plenty remaining in and on the medium to replenish the next grow as soon as they’re rewetted. Bactors like nitro simmonas and nitro bactors can go dormant in a dried out state, but will become “alive’ again as soon as they’re moistened. Of course, you’ll keep a more robust living culture going if you dont allow the used rock to ever dry out completely.

    The only time when a grower would really want to sterilize the rocks is if he had suffered a root disease in the previous grow. A gnat infestation is NOT a good reason to sterilize the rocks since they can easily be treated using BTI bacteria and semi-sealing up the wetted bag (or tub) full of rocks for a week or so to prevent the life cycle from any possibility of continuing, but keep an air stone running in the closed up wet bag or box of rocks to help keep the bacteria colonies thriving.

  •   Last modified: 16:19 – Dec 29, 2004  
    Quicklink:  http://overgrow.com/growfaq/1692
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    faq:1692 “Should I use Lavarocks or Expanded Clay?”