Thoughts on Volcanic Rock as a non-organic ingredient in soil

[WayneByerly]

The flora of Hawaii grows insanely well. To the point of being almost unbelievable. And the greatest item of attribution that I have seen throughout my life has been the fact that they grow in rich volcanic soil. Naturally, this is not the ONLY reason that Hawaiian flora grows so intensely prolifically, but is one of the primary reasons that it does. What i've read/heard says that volcanic rock provides:

1) a large number of mineral supplements not found in any other single ingredient
2) drains VERY quickly,
3) is VERY light weight making it easy for roots to progress through their medium
4) provides terrific aeration
5) it has better water retention than MOST other items because of it's rough surface

Some people cite the fact that cacti SHOULD be grown in their native materials, in order to look natural, while others insist that their cacti be provided with what makes them THRIVE.

So I would like to hear some OPINIONS AND/OR DISCUSSION--pros, cons, indifferences--regarding the use of small volcanic rock or lava rock (1/4" size stuff) as an inorganic part of their soil mixture INSTEAD OF more traditional items, like perlite, turface, small stones, coarse sand, and pumice, most of which are heavier and lack (for the most part) the surface texture that causes volcanic rock to retain water and irregular shape that provides so well for aeration. Is it a superior ingredient? And if not, then what is, and for what reason? Experience does count! Spread some of yours around to the greatest extent possible if you don't mind ... please.

[greenknight]

Lava rock varies greatly depending on how much gas the magma contained - it can be very porous (pumice), totally non-porous, or anything in between. It's all good. The less porous types are heavier, which can help make your pots more stable versus using straight pumice (which is very light when dry). They retains less moisture than pumice, which might be desirable for some species - but it also less air, which is a disadvantage. Some proportion can definitely be used to advantage.

I live near Mt St. Helens, which has produced every type of lava during different eruptions. I tried using the debris flow (lahar) material which was dredged out of the river after the 1980 eruption, which blew off a large chunk of the mountain, contains everything from pumice to very hard rock in various size pieces. I used a 1/4 inch wire mesh to screen out the larger rocks. This stuff worked very well, but it's a lot easier just to buy pumice.

As for using the native materials for your cacti, growing in a pot is a completely different environment.

[DaveW]

Cacti often grow where they do because they have no choice being slow growing, usually to escape competition by more vigorous plants that would quickly overtop and shade them out on more favourable soils. Therefore mimicking similar potting soils does not always produce the best results. However as you say, volcanic soils can be very rich and if you are using volcanic rock to replace inorganic materials it makes little difference what inorganic material you use, provided it is not toxic.

A friend of mine always claims you can grow cacti in almost anything as long as you get the watering right, having seen them growing in habitat over many years. As always it can be if you prefer the "natural" look through growing in soils similar to habitat which retard their growth ("Stone Eater" type cultivation), but remember, as Spence says, soils in small pots do not behave as a large soil mass does in open ground. Some however prefer "show bench" more bloated and green type growth obtained from using more organic potting ingredients.

Growing in purely mineral substrates is often akin to hydroponics where the essential elements for growth are provided by regular fertilisation. However some cacti grow in habitat in similar soils, but seemingly do extract the minerals they need from the rocks they grow in, even though they are never fertilised and may grow a lot slower.

[toadstar]

Crump Greenhouse uses a lot of scoria in the potting mix they sell and it seems to work well for my cacti.