No one can tell you "exactly" what to use because everyone here seems to have their own opinion of what can be used successfully based on their past experiences and current plant cultural habits with respect to maintaining container plants.

With your finding that the topsoil + compost is "heavy" (meaning it really isn't draining the way it should and is compacting), then you may want to pull out your plants and mix in some other things to loosen it up like gravel, perlite, small bark chips, etc. Alternately, you could plant your plants in small pots and sink those into your current tub of soil, covering the small pot rims to camouflage. Otherwise, you may need to just dump the topsoil and replace with something else. If that tub only has the single "drain" as the drain hole, that really may not be enough to get water out quick enough.

I personally use soiless container mix (like Promix) for containers but others use other types of media like coconut hulls, turface, miracle gro soil, their own specialized mix of various ingredients, etc. It's a matter of experimenting based on your particular growing conditions (eg., how much sun, rain, wind, and temperature extremes, etc., that the container is exposed to) and how much effort you are willing to put into maintaining the planter with respect to supplemental watering, fertilizing, etc.

How long have the impatiens been 'sitting there'? Plants often go through a period of adjustment when taken from one place and are put into another. Depending on the plant, what it is put into, the weather and other factors a plant might appear to sit there for 2-3 weeks.

Hi, Bluebell. I've been speaking out for years against using compost, topsoil, garden soil, or fine sand (anything smaller than 1/2 BB size) in containers. the particulates are just too small. The most frequent reason set forth in disagreeing is "It works for me", and that's fine, but by the number of posts like yours, it's pretty apparent it works poorly, at best. It is near futile to try to amend fine soils by amending with larger particles - think of a bucket of sand that you add the same volume of BB's to. An improvement? Nope. It might hold a little less water, but it will still be sorely lacking in O2, which, IMO is a container gardener's most important priority.

Growing in an open mix with larger particulates and plenty of air in the soil exponentially widens your margin for error. As long as you keep the soil from drying, your watering habits become less critical as drainage/aeration increases. You can read a new post on one forum or another every day, bemoaning poor plant performance in soils comprised of the ingredients above, but I've yet to hear a complaint about a well aerated soil that drains quickly, unless it's related to the need to water/fertilize more frequently.

Though no one can offer the Rx for a perfect soil that fits your exact needs, we can surely point you in the right direction and even help you make decisions along the way.

Al

I would recommend you empty out the tub, drill more holes in the base of the tub (or at least put a wick through the hole to help drain out extra water). When you refill, do NOT put rocks or anything else at the bottom, but mix perlite and a hard fired product (such as Turface or Schultz Clay Soil Conditioner) with Promix (or pine bark fines, as Al does). This should make a big difference in your plant growth.

Al is a wonderful resource. I've bookmarked this post of his. If you read it, and understand the science behind it, you will see why you could be having trouble with this "heavy" mix. He also has included a recipe that works for him. You can tailor it to suit your needs, but the basic principles are the same.

Here is a link that might be useful: Al's soil science