I suspect you'll have a lower CEC with Perlite, and I agree that frost cycling will likely have a better chance to break down Perlite, but with those temps, ~15+ cm down in the soil should be adequate insulation.

I'm not really into alpines, but a number of alpine growers AIUI use a product called Oil-Dri (for automobile applications) as a substitute because of cost. Just a thought as I don't know if it is available over there.

Dan

Have you tried googling sponge rock or pumice?

Even I have learned that "perlite" is not even a close alternative to "turface".

Good luck!

Oil Dry works for some and not for others. It must be tested to determine if it reverts to mud when saturated.

If it was me I'd probably up the bark component to 50%. I don't think I'd want only 1/3 of it that would retain water/nutrients.

Pumice in the right size range has proven hard to find -- the closest to spec that I've found so far can be had at the below bonsai supplier (the word bonsai seems to instantly mean 4x the cost around here), and the grain size is a rather large 3-7mm (1/8"-5/16")

http://www.bonsainostrum.com/index.php?main_page=product_info&cPath=7&products_id=2

That said, about 5 minutes ago I learned that there is a fellow who has apparently come across a wholesaler that will allow you access to their piles to screen out what you want. This is especially interesting because no one sells pine bark in any size below 1 cm (I was looking at renting a chipper), so fines in the ideal 2-3mm size range are apparently abundant and quite cheap, simply because very few want them. He mentioned the wholesaler also has pumice and LECA. I am pinning quite a bit of hope on getting the contact info for this wholesaler from this fellow, who is understandably reluctant to give it out.

I guess given the choice between Pumice, LECA and Perlite in the ideal size range at the same price, Pumice would win out due to the highest CEC and Water Retention? I'm not familiar with it's properties, just making a guess. If you have any light to shed on this please do share!

In Hawaii years ago in our fruit tree nursery we used cinder rock (pumice) in about the size range you are talking about with incredible sucess. We never in 10 years of growing used perlite in our mixes. It was way to expensive for our use. We also didn't have acess to pine bark except bark nuggets shipped in from the west coast, again way to expensive. We used soil (not the best choice but it worked for 1 to 2 year grow out cycles when it was used at less than 20 percent. The pumice was about 60-70 percent with soil, peat moss, and composted manures making up the rest with peat moss and manures being the least of the 4 main components.

The CEC was great as I recall and moisture and air ratios were also very conducive to containered tree growing. I would watch for possible high salt (can be leached out if need be). Sometimes the ph was somewhat high but with acid soils and water it wasn't much of a problem for us. It may or may not be for you.

I would love to have access to bulk pumice in your size range by the truckload if I could get it, but all I can find is lava sand and it is just too small in my opinion for anything but small container growing.

I am not sure what LECA is? So I can't help as to it being better than pumice or perlite.

I have used perlite quite a bit here in Central Texas and we have temps much like you describe and over several years of use I have seen minimal degradation or breakdown. The only problem I have seen is in overmixing breaking it down but not much once in the pot. I still have some pots from several years ago that has lots of perlite in the mix and they will be great for raised beds (not pretty but still completely functional. Maybe different deposits around the world have different enough properties that we can't make a apples to apples comparison. I can only speak from my experience with mostly Mexico derived perlite.

Hope this is of some use for your situation.

Good Luck and Happy Growing David