I thought 5-1-1 called for perlite, not vermiculite? Greenhouse Megastore has (or had, the last time I looked) 2 cubic foot bags as cheap as anywhere I found online. But, shipping is a bundle for something that size.

As I understand it, they can be used interchangeably. I used perlite the first time around so I was thinking about trying vermiculite this time.

My gardening math is very poor. Would two 4-cubic foot bags be sufficient for twenty 20-gallon fabric pots using a 5-1-1 mix?

I figure the following.....20x20 = 400 gallons of 5-1-1 mix which is approx 54 cubic feet. Using Al's formula 1/7th of that mix should be perlite which comes to just under 8 cubic feet. Sound right?

I'm far from an expert, having tried 5-1-1 in containers for just one year and promptly abandoning the notion. I have ~2 cu yard bof 5-1-1 I am repurposing. But, perlite and vermiculite serve different purposes. Perlite lightens, loosens and promotes drainage. Vermiculite might lighten and loosen to a degree, but it retains a lot of moisture. Maybe it works as a substitute and I'm just wrong...wouldn't be a first! I'm sure someone smarter than I am will be along shortly!

With fabric pots my goal is to retain moisture as much as I can. More retention equals less frequent watering. At least that's how I see it.

Vermiculite and perlite are by no means interchangeable!! While both are heated mineral products, they act in a soil mix quite differently. Perlite, being very stable and porous, helps to create air space (pores) between the individual soil particles. And it retains water but does not absorb it. Vermiculite will initially help with porosity but it absorbs water to a signification degree - 3-4 times its own volume. And once fully saturated with water, its structure collapses and it then loses any ability to assist with drainage and aeration. In fact, when it reaches this collapse stage (and that can happen quickly with the need for frequent watering of any container soil), it then impedes or slows drainage considerably, making the potting medium virtually unusable.

Whatever you do, do NOT make the mistake of substituting vermiculite for perlite in either the 5-1-1 or gritty container soil mixes!! You will not be happy with the result.

Also, there is a definite limit as to the amount of moisture you want your container soil to retain. If this were not the case, then just using Miracle Gro or any cheap, discount brand of potting soil would work. Both the 5-1-1 and the gritty mix were designed to be of the proper texture and aeration to allow fast drainage yet still have an ability to retain some soil moisture. But if growing anything in a container, you must be prepared to water as needed, not when it is convenient to you!! Sorry :-((

Understood gardengal. I could have sworn reading someplace in these forums that vermiculite could be used in a 5-1-1 mix instead of perlite. I must have read it wrong.

I don't garden in containers, so I'm not well versed in the 5-1-1, however, could it have been Mel's Mix for raised beds where the idea of vermiculite comes in? I'm not saying it's good or bad, but 1/3 of the formula for Mel's Mix is vermiculite - usually the coarse grade. I do think some people interchange the vermiculite for perlite in that mix.

"could it have been Mel's Mix for raised beds where the idea of vermiculite comes in?"

Very likely, although IMO, Mel's Mix is a huge waste of money for any raised bed, square foot gardening or not. You just do not need vermiculite for that purpose nor is the inclusion of 33% peat moss of much value (and tends to overly acidify the mix as well). For any raised bed situation, just garden soil enriched with organic matter like compost is all that is necessary. The elevation of the beds themselves provides all the drainage assistance necessary.

Container gardening is quite different and fast drainage in any container is vital. Perlite, pumice, granite grit and Turface all help to provide that - vermiculite does not!

'The elevation of the beds themselves provides all the drainage assistance necessary.'

I think a lot of people forget this and think they need to supply good drainage through their mix as well.

Vermiculite has a completely different shape to perlite. If you Google vermiculite you will see its mug shot. It is not round, like perlite. It will not hold air between its particles.