What you have are seedling chestnuts. Even if the 'mother' tree was Colossal, you don't have any clue as to what was the pollen parent. It's a genetic crapshoot as to whether they'll be as good, better, or worse, than the 'mother' tree you collected them from.
As to whether they'll do well in St. Louis, it's anyone's guess.
Skip the amendments...if you have, as you've stated, heavy clay... you'll be creating a 'bathtub' effect that will tend to hold water and may cause some root rot issues. They have to grow in your native soil at some point - might as well get on with that task from the outset. If you need amendments, just apply them to the surface, as mulch, and let the worms and soil microbes incorporate them into the soil.

Now...others may differ with me...I like chestnuts, but I would not want chestnuts on a small residential lot. Have you ever smelled them in bloom?(look it up) Do you know how spiny the burs are? I like to walk around barefoot in my yard from time to time...but not if there are remnants of chestnut burs about... sometimes even tennis shoes are not sufficient protection.

Thanks--that's all good advice. Sounds like I should just stick them somewhere in the woodsy areas around here where there's a reasonable spot to plant, rather than anywhere in or around the yard. If I don't have to amend and am just going to leave them to their own devices and see if they can make it, that should be pretty easy.

Bear in mind, I wasn't trying to discourage you - but did want you to be aware of potential issues that you might not have known about.

I have chestnuts here - but nowhere close to my house/yard. Know some folks who rent a home in town with 3 Chinese chestnuts as the only 'yard' trees; they curse them at least twice a year(and possibly every day) - when in bloom, and when they're dropping burs.
Nice trees...but not something I'd want close to my door.

Cold hardiness SHOULDN'T be an issue - I think Japanese are the least tolerant, but in zone 6 you should be ok. I have Chinese growing in zone 4.
They prefer acid soils and will do well even in REALLY acid 3 or 4 pH, but mine seem to like my 6.5 clay just fine.
Best siting for chestnuts in clay - find a sloped part of your yard that won't pool water. Find a sunny spot, the sunnier the better. They won't die in shade, but they will languish and grow MUCH more slowly. And finally, as folks above have said, not near somewhere where there is a lot of foot traffic.... ouch.

Are they very drought tolerant? Although it tends toward clay, we are generally fine on drainage if we don't plant in the low spots--where I want to do that I've generated elevated beds first anyway, to make otherwise unplantable spots suitable for roses etc. So in a sloped or more elevated spot the soil would certainly drain well, but it might be quite dry in summer. Maybe the first year I could go check and give them a drink. The places I'm thinking of as possible would have lots of sun in that case.

I had our soil tested a few years back and it ranges from fairly neutral to slightly alkaline, so that may be okay. I've mixed in additives like elemental sulfur for the occasional plant that liked it more acidic as well. People do grow azaleas all over around here, so most places probably are not too hospitable to plants with a preference for acidic soil.

From what I have heard, chestnuts need water when young, but once established, they become drought tolerant. Saint Louis is fairly far east - you should be ok. Just be nice to them for a couple of years.
I wouldn't do elevated beds because the trees will outgrow them. Plus clay holds water better than sand anyway, so a slope should be fine. You are just trying to avoid flooding out the roots.

that we can do--spouse thinks we should put them off in a corner (should be sloping and very well drained) where we won't end up walking on the nuts but they are close enough to be put on the drip watering system to keep them happy the first few years. Thanks for all of the advice--glad to have a plan for the little guys!

If you were hiking, ie: in a wild area, I'd offer the opinion that you have a California buckeye, not a chestnut. Do you remember if the nut was shiny, as opposed to slightly fuzzy?

The buckeye, is a very nice large bush, or if pruned small tree. The flowers are very nice and fragrant. However, the nuts are toxic. (As mentioned in the hopefully linked Wikipedia article) I straddle the fence on this plant. We have a large one behind the house, where I don't have to worry about the horses 'experimenting' on the nuts, (and they might be unpalatable enough that it wouldn't be an issue. The squirrels don't touch 'em either.) the nuts do tend to easily sprout, and the resulting seedling forest is a PITA to deal with along my fenceline.

Here is a link that might be useful: California Buckeye

well, well, well...isn't that interesting! I walked in from our hike and showed these to my father in law (who's lived in Monterey most of his life) and he identified them as chestnuts so I thought nothing of it, but I definitely think you are correct.

The green pods had opened to yield these dark brown smooth nuts of about 2 inches that were beginning to germinate (and rooted nicely in my pot) looks just like what is on your page and that would make more sense. I'm going to try to attach some pictures. I bet these were the beautiful silvery branched small deciduous trees I was seeing everywhere as well. I checked another page and they end the article with:

"Though thoroughly inedible (unless leached of their toxins, as the Native Californians did), there is something irresistible about this seed, looking as if it had been carved, lacquered, and polished; few can resist picking up one or more, often pocketing them to be brought home for a show-and-tell with family or friends. And then the rains come, and the year-long cycle begins anew. - See more at: http://www.pacifichorticulture.org/articles/california-buckeye-a-tree-for-all-seasons/#sthash.zsT2m7jo.dpuf";

I was definitely one who couldn't resist pocketing a few...

In this case, I'd be dealing with much smaller trees/large bushes, which would be better for this spot, but we are zone 6a (changed from 5b as climate change has happened over these last decades). So I may lose them if we have a harsher than usual winter--we had one of the 10 coldest winters here in 2013-14 and I did lose some variegated hollies that are more suited to Great Britain but had been on the lot for about a decade. But it is still worth trying since we are in the range they can take and would be fun to see what they can do.

Sounds like the plans for a well draining corner spot where the acorns don't end up in the yard and I can keep them watered the first couple of years and they get reasonable afternoon sun, would still be just about right. If they end up establishing, the seedlings do sound like a bit of a pain, although I already have landscape fabric and mulch through that whole area to keep all of the other non-native stuff (like the asiatic shrub honeysuckle and trumpet vine) from getting a firm foothold. So pulling weeds along the lot line is already a part of my life.

Thanks for the information...I guess these are the hazards of collecting 'magic beans' along a hike. It has already been fun to watch them growing in my pot.

Yep. Those look a lot more like our local buckeyes, than the chestnuts I'm nursing along.

I was just a little concerned about potential "chestnut" taste testing in the future. They are, as I said, a decent looking bush, and I do enjoy the scent when they're in bloom. Good luck with them.

Thanks for the heads-up....we'll definitely know not to taste test :), should we be lucky enough to keep them going. I don't see instances of these being grown outside the Pacific coast, so maybe they will greatly dislike our hot and humid summers and colder winters, but worth a try. Knowing they won't get so big gives me a couple of potential spots to stick them where they have a shot, once the danger of frost is over. Thanks again!