I would think that it largely depends on the brand of soil that you buy. I cant speak on the exact ph numbers but I think that most soils claim to shoot for the range in which most plants would thrive (5.8-6.5). Those are not my numbers by the way. The amendments in the soil would probably be a good place to begin venturing a guess.

For example, Fox Farms Ocean Forest says that it has Oyster shell flour (I think thats the way it reads) added to it for balancing ph. Oyster shell flour/powder whatever you want to call it, is mostly comprised of calcium carbonate (some where around 90-97%). Calcium Carbonate effectively nutralizes ph most of the time but those numbers can get skewed easily by other amendments. Another amendment commonly seen would be dolomite lime which does the same thing but has a bit more magnesium (not always a good thing in my opinion). There are many "ph buffering" amendments that most soil companies use but those are the ones that I tend to hear the most about. I hope this helps you out some.

A few things to consider:

Soil is a heavy commodity so it will be 'produced' and packaged close to the point of sale => actual contents will very considerably by region.

There are no real standards (at least in the US), so you can get everything from dredged river sand to farmland topsoil to...whatever.

Some products (the more expensive) may add additives such as organic matter and even fertilizers. Others do not.

If compost is added, and it happens to be derived from barn or CAFO manure where the facilities were treated with lime, the pH could be higher than neutral. OTOH, a premium product with additives is more likely to have been tested for quality control.

Bottom line, it's a crapshoot, but in most cases, it's *probably* reflective of regional soil conditions.

Tox is more corrector. There are no standards for pH in bagged potting soils, or any other soil mix you purchase. The only source I could find that did mention potting soil pH state that would be between 6.0 and 6.8 pH. Keep in mind that potting soils are made of acidic materials, peat moss, finely ground bark. etc.
Maybe some manufacturers of potting soil wouild add buffers but unless it is listed on the ingrediants label lime, or something, will not be there.

I just reread the orig. post and it was about bagged garden soil *and* potting mixes. My answer was directed at bagged 'topsoil' and to some extent 'garden soil' mixes. Potting mixes are a different thing as kimmsr points out. Thanks for clearing that up.

"There are no real standards (at least in the US), so you can get everything from dredged river sand to farmland topsoil to...whatever."

O yea maybe with topsoil, but bagged potting mixes are perfect ph, consistently the same ingredients everytime. For 10 bucks for 2 cuft, they better be good or else another brand of potting soil will see this and step up and make a better cheaper potting mix, O wait, They did!!! The NEW Hyponex from scotts NOT the black bag. The new green bags sold at 8.27 for 2 cuft are better then any other potting soil I have seen and cheaper. It is about time they gave a good potting mix at a good price!

Sorry justr had to point that out!

I did have very good luck with a couple of big tomatoes grown in Scotts MG potting mix this year, I have to say. Course I got most of it free from the dumpster out back, torn bags... :-D

Pro-mix is pretty good - Peat, perlite, some bark, and wetting agent. The only problem with peat-based potting soils is that after a year or two the lime leaches out and the peat breaks down, giving you a very soggy, low-PH bog-in-a-pot. If I am growing something in a pot for a long time, I mix in pine bark fines and coarse sand, plus a bit of slow-release fertilizer. But, if you are only growing annuals or perennials in a pot for one season, the PH is around 6.5. It slowly drops afterwards.

I agree. I reuse my soil once. I take the soil and add some peat, perlite, and lime. After that it is about done and it goes in the garden. So 2 years is what I can get out of potting mix. I use only synthetics so maybe the micro activity is not present to break down the media quiker.