Perlite is superheated volcanic glass. When it is heated it expands like popcorn and becomes porous. It does retain water but how much is dependent on the sizing.

And to maintain proper aeration with any kind of material you add to a container mix, you want to avoid very finely textured stuff, like peat or coir if finely ground. they are what creates lack of porosity, not the perlite. The more coarsely textured the ingredients the better the porosity and the faster the soil mix will drain and the longer it will last without replacement.

So is perlite not used for aeration then? Because that's the #1 use I keep reading about. Yes, I fully understand what you're saying which is exactly what I suspected.

If this is true the only good Perlite does is make soil lighter (by weight).

The fact that the material itself is porous of course will add to aeration!! And the rough, large texture helps to maintain porosity/aeration in the soil mix.

Al's many technical posts on this forum should have explained that pretty well :-)

Thank you for your time. I know asking a question about perlite may seem very basic, and it's better to keep things simple for someone new, but I am very familiar with Al's technical posts about water retention and its principles. Your last post actually contradicts your first in this regard. If one were to bury a piece of perlite to be surrounded by particles of soil less than 1/16" then any qualities of aeration that perlite once had is now gone. 1/16" particle sizes isn't dust or finely ground either. But your first post is correct in that regard.

I understand perlite is porous and comes in many sizes but I doubt (unless someone is growing strictly in perlite) people aren't using so much perlite that no soil surrounds it. I tried to make this clear in my original post because so often answers here default to basic understandings. I was trying to dive deeper.

"Your last post actually contradicts your first in this regard."

Actually, there is no contradiction :-) I've been doing this a long time and actually teach classes in it. Perlite in itself is porous and because of its bumpy, rough texture, it contributes significantly to aeration. What you want to avoid are flat, smooth, solid surfaces.....which the perlite lacks. Pumice works in a similar manner

Lack of aeration in a container soil mix is directly related to 1) the size of the particles of the materials involved, and 2) how easily those organic components breakdown and collapse. Because both peat and coir are very finely textured (as is any garden soil), they tend to be the primary causes of loss of aeration in a potting mix. And they both degrade in a relatively short period of time and become smaller and smaller and fill up the pore spaces in between the other media ingredients.

If you have issues with declining or lack of good aeration in your potting mix, don't blame the perlite!! Blame the very small particled, finely textured other stuff. I have permanent container plantings in the 5-1-1 mix that can last as long as 5 years before aeration becomes an issue. But the 5-1-1 has a very small percentage of finely textured ingredients.

Thanks again for your post. 5:1:1 is a completely different animal for reasons you and I both stated. But I don't use 5:1:1 for seedings to go into my garden. Perhaps I should, do you have luck with 5:1:1 and seedlings?

I was speaking specifically about Perlite's typical use which is that as a soil amendment for common soils like compost, peat, and literally everything you buy at a store and 99% of the population use.

Yet again you confirmed using such soils are the real issue here. And since you understand water retention issues you surely agree if you take any one of those soil structures (which certainly are smaller than 1/16" in particle size) and surround a piece of Perlite with it, then the retained water would displace much of the existing air the Perlite is providing and further surround the perlite itself with retained water.

That is unless the soil itself is exhibiting some sort of force field to avoid it from ever coming into contact with Perlite causing the water to be retained onto it. Forgive me for being sarcastic. I would assume you have more knowledge on the subject than I but am a bit confused why I have to spell it out to find a satisfactory response.

Seth, I'm not clear what use you are referring to, in containers or in the yard in real soil. My view is using perlite outdoors in real soil is a waste. Drainage issues are different and there are far better and cheaper ways to amend soil.

Now perhaps there are specific outdoor uses where it makes sense but I'm speaking in general.

Yep seth, perlite is a waste of money and I do about the same as you. Never used the stuff, never will and never had a problem. Joke IMO, someone said lets come of for a angle to sell this worthless stuff and BINGO! Now everyone uses it.

@armoured - I am speaking specifically about container gardening which is why it was tagged (years ago this would have been in that specific forum so that would have been more clear)

You are correct, when connected to the earth there is less need for aeration because of the mechanics of water retention (as pointed out by Tapla), although perhaps Perlite would be useful to lighten the soil.

@outpaientzero - this second look at Perlite is worth considering and I believe you're not missing as much as we've been lead to believe. Unless i'm wrong, and I'm still waiting to be corrected, mechanically it can't function as described being a powerful aerator amendment unless the issues of soil particle size has already addressed which then makes the use of Perlite redundant. It would, however, be a great inert addition if that's the intended purpose. And it would lighten the soil if compaction is the issue.

I'm not claiming to be the expert. But it doesn't add up when you consider the experts own opinions.

I try to stay out of the Container Wars - my mistake. Not clear to me what you're arguing about, so do carry on.

@armoured - I don't believe I could have spelled it out any clearer than my first and every subsequent posts. But this seems to happen often on forums. Until someone knows differently I'll assume I'm right.

I got it, he's addressing the myth perlite helps aerate soil and Seth has his reasons why he doubts the claim, and so do I.

Soil aeration is gas transport in the soil through diffusion from a higher to lower concentration. Sounds good so far, and soil porosity is the most important but so is tortuosity to allow diffusion. And Seth's argument is each grain of perlite is encapsulated with other matter eliminating tortuosity and therefore diffusion.

And by golly Seth is right. The diffusion and aeration Requires greater tortuosity than perlite offers and not at the macro level. It must take place in the macropores of the soil where the plants grow, and not in and along side perlite only with tortuosity through out all the growing medium. So now I'll refer to Cornell University's guide lines which suggests a mix of 2 parts peat to 1 part perlite for flowers and 50/50 mix for vegetables. You may as well do hydroponics because all your nutrients won't have time to cling and ends up at the bottom. I've never seen a pot from any growing center where perlite is even close to 20%, and you know why? Because they use it for moisture retention and not aeration. If anyone mixes it 33%-50% speak up but the idea of perlite for aeration is a placebo and myth, which can easily be replaced with Peat or Coco Coir for moisture retention.

I guess I wasn't sufficiently clear - by 'do carry on' I meant 'without me.'

@tropicofcancer (6b SW-PA) - this is exactly the kind of helpful response I was looking for. It's good to know I wasn't out of mind when considering all this, and that others have already looked into it! The 5:1:1 explanation is equally helpful. Much appreciated!

Thanks also for the thread Tropico. I'll give it a better look tonight but it all makes sense. I was a little hesitant responding to avoid a battle but I have a hard time accepting "because it is written and therefore it will be" as valid and binding.