The pine bark thing is hit/miss. You often find it as PB Mulch, PB Fines, Soil Conditioner, Clay Soil Conditioner ...... you're looking for a good size, not a name. It should be CONIFER bark - preferably pine or fir, and not hardwood bark or composted 'forest products'.

The 'concept' behind the soil is to reduce perched water so the roots can always breathe - not just after the soil has had a few days to dry a little. The latter scenario kills roots the plant has to grow again - a decidedly expensive thing in terms of lost energy. Adding EXTRA compost or other fine ingredients defeats that purpose. If you do that, you simply create a soil with the same drainage characteristics as the soils you're trying to avoid. I limit fine particulates to no mire than 1/6 of the o/a volume. IOW, if you have 5 parts of bark and 1 part of perlite that's 6 parts. Add 1 part of peat OR compost and it's 7 parts total, with the fine ingredients making up 1/7 of the mix. The fines in the bark add to that 1/7 making it closer to 1/6. Adding more fine particulates increases water retention and reintroduces the perched water you want to do without. Are you following me? You can always add water retention to a soil by adding fine particles, but you CAN'T always add aeration by adding large particles. You have to START with large particles to reap the benefits of a truly well-aerated soil.

I'm saying this in the kindest way because I labored hard to understand the very same concept as it relates to bonsai design. You need to thoroughly understand the rules before you can effectively break them. The rules, in this case being the rules of physics as they relate to water retention in soils.

Vermiculite is very unstable and adds significantly to water retention. I rarely ever use it in soils, choosing perlite or other large particulates to adjust (almost always decrease) water retention.

Al

thanks al, so would it be ok to use compost in place of the peet?

Yes, but using too much negates the benefits of the large particles.

Imagine: You have a quart of marbles in a quart jar, so the jar is full of marbles - right? You can still add a pint of sand to that jar of marbles though - right? So how do you get a quart and a half in a quart jar? ;o) You know that drainage and aeration was awesome when it was just that quart of marbles - yes? Yet, by restructuring the 'mix' so only 1/3 of the o/a volume of the mix is comprised of fine particulates, you can see in your mind's eye that the entire mix took on the exact drainage characteristics of the sand only. The marbles have no effect on drainage (flow-through rates) OR on the ht of the PWT. After the sand is added, the ONLY thing the marbles do is take up space that MIGHT otherwise be occupied by sand + water, thus it simply reduces water retention w/o adding aeration or lowering the ht of the PWT, both of which would be remedied if the soil was more than about 85% larger particulates.

This also explains why those suggesting that you can simply take a peat-based bagged soil and simply add more perlite and some bark to improve it. It won't .... for the same reason the marbles were ineffectual at improving drainage and aeration. In that practice, the bark does very little or nothing to improve aeration (just like the marbles) and it is very ineffective at reducing water retention because it holds almost as much water as the peat. The only significant benefit comes from the fact that the perlite reduces the soils ability to hold water. Going totally technical, the only perlite that is effective is that which is occupying space WITHIN the PWT because all the soil above the PWT would be sufficiently aerated and wouldn't need the perlite.

The benefit from bark and perlite comes not from the materials themselves, but from the air spaces between the particles. If your amendments aren't increasing aeration, they serve little purpose & you may as well stick with a bagged soil because you're not changing anything of significance until, again, the large particulates are more than 80% of the mix.

Al