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

As I understand it the point of using porous materials is not to increase/decrease water retention on the outside of the particles, but to provide sufficiently large pores to allow air circulation around the roots while still providing enough humidity (from water stored in the inner pores) to help the fine roots to stay alive. That's the point of using products like Turface or Floor Dry. I'm just wondering if pumice is really in the same category. If you look at xerophyte nyc's experiment here, you'll see that a similar substrate, lava rock, holds very little water. (Thank you for that, xero!) The pores in volcanic rock may be so poor at storing water as to make these substrates comparable to granite, a substrate used for its minimal water retention.

You can start by reading something I wrote a while ago, then posing any questions you have: Though roots form readily and often seemingly more quickly on many plants propagated in water, the roots produced are quite different from those produced in a soil-like or highly aerated medium (perlite - screened Turface - calcined DE - seed starting mix, e.g.). Physiologically, you will find these roots to be much more brittle than normal roots due to a much higher percentage of aerenchyma (a tissue with a greater percentage of inter-cellular air spaces than normal parenchyma). Aerenchyma tissue is filled with airy compartments. It usually forms in already rooted plants as a result of highly selective cell death and dissolution in the root cortex in response to hypoxic conditions in the rhizosphere (root zone). There are 2 types of aerenchymous tissue. One type is formed by cell differentiation and subsequent collapse, and the other type is formed by cell separation without collapse ( as in water-rooted plants). In both cases, the long continuous air spaces allow diffusion of oxygen (and probably ethylene) from shoots to roots that would normally be unavailable to plants with roots growing in hypoxic media. In fresh cuttings placed in water, aerenchymous tissue forms due to the same hypoxic conditions w/o cell death & dissolution. Note too, that under hypoxic (airless - low O2 levels) conditions, ethylene is necessary for aerenchyma to form. This parallels the fact that low oxygen concentrations, as found in water rooting, generally stimulate trees (I'm a tree guy) and other plants to produce ethylene. For a long while it was believed that high levels of ethylene stimulate adventitious root formation, but lots of recent research proves the reverse to be true. Under hypoxic conditions, like submergence in water, ethylene actually slows down adventitious root formation and elongation. If you wish to eventually plant your rooted cuttings in soil, it is probably best not to root them in water because of the frequent difficulty in transplanting them to soil. The brittle "water-formed" roots often break during transplant & those that don't break are very poor at water absorption and often die. The effect is equivalent to beginning the cutting process over again with a cutting in which vitality has likely been reduced. If you do a side by side comparison of cuttings rooted in water & cuttings rooted in soil, the cuttings in soil will always (for an extremely high percentage of plants) have a leg up in development on those moved from water to a soil medium for the reasons outlined above. ***************************** Some key issues in determining whether or not cuttings will strike are the state of health/vitality of the plant material the cuttings were taken from, whether or not there are disease organisms in the rooting medium (all unsterilized peat-based media does), how well-aerated the rooting medium is in relationship to how deep the cuttings are stuck. Al

There is a 'threshold' based on size that in large part determines how much change you can expect in soil porosity, which is as essential as moisture retention. Imagine a 2L flask half filled with peat, and a 1L flask filled with 20mm BBs. In your minds eye, you can see all the wonderful air spaces between the BBs. Not so much between the peat particles. If you thoroughly mix the BBs into the peat, what do you see? You see ALL the air spaces between the BBs completely filled with peat and the o/a air porosity per given volume diminished significantly. It's only when the volume of BBs is so large there is not enough peat to fill all the large spaces between the BBs that we see any notable gains in air porosity. The images below of the 2 types of media I grow in embody consideration of that 'threshold' concept to reduce water held between soil particles. If we were theorizing what a perfect container media might be, it's difficult to argue against the idea that physically, the soil would hold all of it's water inside and on the surface of soil particles, and at the interface where soil particles contact other particles, leaving the macropores between soil particles full of air, even at container capacity (a measure of the amount of water a soil holds when it has been fully saturated and passive drainage (caused by gravity) has just concluded). The only way to achieve this, or even come close, is to start with a large fraction of coarse material like pine bark or any of several inert ingredients and be cautios about the volume of fine material you include in the medium. Al