I'm interested in this too. I started a few things and found really quick that it can get expensive to fill large containers.

look around for local suppliers of suitable landscape materials. Depending on what is available, there are all sorts of recipes. Rice hulls, fir bark, coconut husk chunks, gravel, you can make a medium out of almost anything. Small bark to add to the promix would be a good start.

Mix your own. 1/3 compost, 1/3 spag peat moss, 1/3 aeration (pumice, barks, perlite, rice hulls,etc) then add some organic meals (biotone, gardentone) if you want to keep it organic. Good quality compost is key.

" Good quality compost is key."

And yet there are many - certainly on this forum - who are of the firm opinion that compost, if added at all, should be included in a potting mix in very minimal amounts. Perhaps reading through this container soil tutorial will help with a better understanding of what constitutes a "good" container soil, but purchasing the specific ingredients in bulk and mixing your own cuts down on the expense of bagged potting soil considerably.

Compost has high humus levels and high CEC, it is not used specifically for moisture retention. High CEC stores more nutrients longer and prevents PH swings. You will have a more alive soil food web and it will not be muck if you use at least 33% aeration material. For organic vegetable gardening I would use a good amount of compost or EWC or a mix of the two but EWC is not cheap.

Plus OP said they intend to use fabric pots which tend to dry out MUCH faster than solid wall containers.

Thank you very much for the advice. What are CEC and EWC? Most of my containers are fabric also.

Cation Exchange Capacity (CEC), there are a lot of detailed explanations if you want to search for it and read up but basically think of it like a nutrient hopper (more to it than just that). The higher the CEC values of a mix the more nutrients the "soil" is capable of storing/holding onto. Thus making the growing process more of a breeze, you don't need to constantly "feed" your plant/add nutrients, you can just sit back and add water until the plants tell you they need more (top dress). But as others have pointed out this doesn't seem to be the forum for that mindset.

Not all compost is created equal, I'm not talking about using store bought bagged compost, I'm talking about home made compost or vermicompost, aka EWC or earth worm castings, or simply worm castings. You don't need to necessarily make it yourself but I would look for some local sources.

Recent research regarding container media and CEC is suggesting that CEC is largely irrelevant.

That may be true for ornamentals or if you want to constantly have to feed vegetables, then go ahead and use a low CEC soiless mix...but you are off topic at this point. OP is asking for advice on a SOIL mix for vegetables in large containers....go ahead and wash away most of your money right out the bottom of your container....

Everyone washes money out of the bottom of the pot due to leaching. The question is how expensive are those nutrients, and those organic nutes might as well be gold plated for how much they cost per pound of nitrogen. If someone wants to save money on container plants, the easiest way is to avoid growing "organically".

So I checked Home Depot and fish emulsion is about $34/pound Nitrogen and Miracle Gro is about $4/ pound of Nitrogen. Who is the one washing money away again?

I stay away from big box stores, alfalfa meal (nitrogen) can be had in 50# bags from animal feed stores for less than $20 and a little goes a long way. You can go harvest wild stinging nettle and make your own teas out of it and they contain many micro nutrients as well on top of being a great source of nitrogen. Another good nutrient accumulator is Comfrey...you can grow your own and it provides huge amounts of biomass to use...its almost impossible to kill off, use the dried up pulverized plant (top dress) or again make your own teas out of it. Compost can be had for free with some time and planning from yard and food waste. Tomatoes can be grown in 100% compost only and still do well though I don't recommend that. Never did I recommend going to Home Depot as a go to source, you're suggesting miracle gro and bottled products which is mostly water. I'm suggesting sustainable organic gardening. I'm done here...you obviously know more about this subject than others. Good luck OP!

Back to the original question: a cost effective soil mix for a new container gardener. I follow the advice in the discussion gardengal linked to. It's unlikely that a person growing vegetables for the first time has access to real, finished compost. If you can find pine bark fines and make the 5-1-1 mix, your mix will be much cheaper than anything you can buy. I figure I spend about $3 a cubic foot (about 7 gallons). My container garden of about 20 very large fabric containers requires about 50 cubic feet of potting mix. That's pretty expensive even with a homemade mix. I built up to this amount slowly so now I can refresh the pots with 2/3 old mix and 1/3 new mix each year. My advice is to start with a less ambitious list of veggies. You won't get much bang for your buck growing peas or potatoes in a container, and squash and melons require really large containers. I'd stick to tomatoes, cucumbers and beans the first year.

"It's unlikely that a person growing vegetables for the first time has access to real, finished compost"

Why in the world would you say that?? Bagged compost is available virtually anywhere. And just because it is a bagged product and not necessarily 'home grown' does not mean it is not properly finished and of decent quality.

Not that compost in any quantity is necessarily a good thing to add to a container mix.......but it most certainly IS very widely available.

I was specifically reacting to the comment by thegardenergardener:

"I'm not talking about using store bought bagged compost, I'm talking about home made compost or e that aka EWC or earth worm castings, or simply worm castings."

The only quality compost in my area that is not home made comes from organic farmers who sell in bulk. there is a county 20 miles from me that composts yard waste, but it has been shown to contain pesticides from treated yards.

With commercial products, liberties are taken with the word "compost". I wouldn't go near bagged compost unless I knew exactly what was in it, and especially that there was no sewage sludge (aka biosolids) mixed in. Here's a basic article dealing with that issue:

http://www.motherjones.com/blue-marble/2011/04/your-organic-compost-really-sewage-sludge-rosario-dawson-kellogg-amend

Then there's the matter of pesticides, how do you find compost that isn't full of them. As Ohiofem mentioned, municipal composts can contain pesticides, especially when using material from lawns (that are often heavily sprayed). One way around that is to only use municipal composts during the time of year when lawns are dormant and not being mowed. It's usually free and they often sell compost bins at discounted prices.You can also make your own compost bin with metal poles and chicken wire.

About an hour's drive from my home there's a facility that makes compost from horse manure (66%) and ground up wood pallets (34%), composts it for 30 days at 160F in 15 ft high steaming mounds. I filled up the back of my pickup for $30 a few years ago. But then I started thinking about the medicine given to horses on a regular basis (deworming, etc...) and the fly control spraying that must be done at large horse farms. That's when I started my own compost piles. Weeds are plentiful and easy to pull in early spring. Trees drop copious amounts of leaves, and I have some large oak and pine trees. I found a coffee shop that uses only organic coffee, so plenty of coffee grounds. And I found an organic vegetarian/fruitarian on the Freecycle who was looking for someone to recycle his scraps...every two or three weeks I get 3 5-gal buckets full.

This may sound like overkill for someone with 3 tomato plants, but if you want to garden organically it's not hard to make your own compost. I also added red worms to my bin/piles and let them thrive so I also have lots of the above mentioned EWC.

BTW, I also use Miracle Gro...it's cheap and easy to use. I'm beginning to mix my own potting soil (I have many 5/15 gal containers for vegetables and fruit trees)... I'm adding about 1/3 compost, 1/3 bark fines, 1/3 old container potting mix(which is mostly peat) or SuperSoil, some gypsum, and pumice when I can get it cheaply. BTW, you can buy 1 gal of fish emulsion at Home Depot for $13.50. A gal of water weighs 8.34 lbs, a gal of fish emulsion weighs a little more than that. So the cost is around $1.62 per pound.

jbclem, remember that the fish emulsion is only 5% nitrogen by weight so it costs about $32/ lb of nitrogen.