I don't know much about it but I have seen healthy discussions going on in the succulents forum on this topic.

Pumice, or volcanic stone dust, Perlite, and Vermiculite may serve the same purpose, providing drainage in potting mixes, but bio char will not be the same thing.
Perlite and Vermiculite, in Hawaii, probably are not too sustainable, requiring a lot of non renewable resources (which also produce carbon emissions) to get there.
How useful Pumice might be would depend on how fine the particles are and most likely no one has though of using that abundant resource in place of Perlite or Vermiculite, commercially, before.

Kimmsmr, pumice has been widely used as a soil amendment for decades.

Look for pumice graded to about 1/4", plus or minus 1/8". It is often used alone in growing bonsai, or in various mixes such as 70/30 pumice/peat or 50/50 pumice/peat.

It is more durable than perlite; it adds more both in terms of aeration and water retention (and subsequent release) than does perlite. A 1:1 substitution for perlite in an existing mix recipe would result in a more moisture retentive soil, but also one that still drains well. Whether that is ultimately good or bad for the plant would depend on the other ingredients in the mix, the particular plant being grown, the watering habits of the grower, and the climate/environment.

I haven't typically lived in areas where pumice is available. I did recently try Growstone #2 - the smaller size, up to about 3/8" - and found that it is not a substitute for pumice since it holds little water, but it is a substitute for perlite at 1:1 in most of the common soil mixes.

Growstone tends to be pretty alkaline so rinsing well before use is recommended. It's alkalinity can be offset by the acidity of peat, depending on the ratio of peat to Growstone. It's somewhat sustainable in that it is made from recycled glass, and not so much so in another sense as if you are in Hawaii we are talking about significant carbon footprint due to having to ship it out there.

Pumice, however, should be easily available in Hawaii in a variety of grades. Give it a shot.

This post was edited by zensojourner on Wed, Dec 17, 14 at 20:54

Pumice is an igneous rock formed during volcanic explosions and has been sold for a number of uses including replacement of sand to mix in soils to "improve" drainage. Like sand you need enough, 45 to 75 percent by volume, while organic matter does a better job with much smaller amounts of material.
When someone touts the benefits of pumice ask also how much organic matter is needed to get those benefits. I have been places where this material is laying around with no plant growth until organic matter is added.

Since the thread was about soil amendments, there's an assumptiom that the other components (especially organic matter and nutrients) are there and we're just talking about modifying physical characteristics. I guess it doesn't hurt to make it clear.

I would think there are some gardening people in HI who know about the local materials. Ag Extension, or master gardener's group?

It would be quite a challenge living there and having to constantly choose between what's available locally vs. the high cost of imported *anything*.

Of course. Like every other state Hawaii has a university that is part of the Cooperative Extension Service of the USDA.

Pumice is part of the mineral portion of soil, and in Hawaii is probably what most of the soil is composed of. Pumice has been used to replace perlite

Here is a link that might be useful: University of Hawaii CES

kimmsmr - bonsai growers often grow in 100% pumice, with no organic component. Such trees are usually fertilized with a foliar spray. I would suggest that if you don't see plants growing in pure pumice in the wild, its a matter of available nutrients and has little to do with pumice as a substrate per se.

Oregon State University has done studies with 10%, 20%, and 30% pumice (and the rest is bark) with good results.

There is a grower who recommends and uses a 70/30 pumice/peat blend with good results.

So there is a very wide range of pumice-organics ratios that work well. It ultimately depends on the plants themselves and the watering/fertilizing methods used. Pumice blends that contain organic material (such as bark) will still require fertilization/nutrient supplementation.

But the original posting was about pumice as a replacement for perlite. The thing to keep in mind is that pumice will have a higher water holding capacity than perlite. It is likely it will have a higher air filled porosity as well. It's hard to say exactly how it will perform without knowing what is going in to the rest of the mix.

If pumice is hard to come by in Hawaii - which would surprise me, as I've been told by folks who have lived there that its not hard to find - try scoria. Scoria usually holds a little less water than pumice, but more than perlite. It will be denser than perlite so it weighs more. Of course pumice is also denser than perlite, but scoria is usually denser than pumice, too. The trick might be to find it in an appropriate size.