This question is applicable to a lot more than edible crops, but I'd
like to understand better about potting soil, whether for starting
plants or for container gardening. I've been using the cheap Lowes
potting soil for a few years. It looks to be largely organic stuff.
That is, it isn't just garden "dirt". OK, so that sounds right. Nice
consistency coming out of the bag. But I notice that this stuff
basically turns into mud after a while. That is, after a long and very
hot summer, when stuff is getting watered every other day to keep it
alive, you dig out a plant, and find that the plant has been trying to
grow in highly compacted and hugely soggy soil. This in a pot with
excellent drainage. Just to get the soil out of the pot, you can't do
it with fingers. You need a trowel to excavate it.

Now, this doesn't seem like the kind of stuff plants would like to be
growing in. I should have realized this was happening, because during
the summer, when I watered, it would sometime pool up in the top of
the pots, and take an hour to drain!! The way I was taught, soil
should absorb water, but be permeable enough to let it soak through.

I found a number of fat worms in it, so it can't be that bad, but ...

OK, my bad, using cheap potting soil. Butt head against wall, etc.

But I still have a few bags of it, and it seems to me that with
appropriate amendments, this stuff could still work. What should I
use? Coarse sand? Perlite? Vermiculite? Compost? Peat moss? Now, if
I'm going to add organic material to it, I'd like it to be stuff that
didn't eventually turn into the organic paste I seem to have ended up
with.