Begonias dying? Please help!

[marzz]

Hi guys,

I was hoping you could please help me. I have two balcony containers with ivy, begonias, and violets. In the last couple of weeks, I noticed that the begonias (one in particular) have been very weak. The leaves have stopped growing, faded somewhat (not necessarily dry, but weak and faded), and the blooms keep wilting (the edges brown and few new blooms are forming). They are tuberous begonias, the ones with the smaller blooms. I tried to upload some pics, but no luck. Oh, and they're on a balcony ledge facing east.

I have spoken to a couple of local flower vendors, and they both told me that it could be a result of the (lots and lots) of rain we've been having lately (I live in Montreal). But I didn't get it, as the begonias in the second container get the exact same treatment, and they're flourishing. So it occured to me that it could be the soil. When I was potting these, I ran out of the good soil and added some cheaper stuff I had purchased for something else, then mixed it with peat moss to fill up the containers properly. So I am now thinking...

a) the soil is too heavy as a whole (I've noticed that the "sick" pot doesn't seem to drain as freely when I water the plants);

b) I added too much peat moss and the soil is too acidic (not sure if this is related, but there is a green mossy film on the top of the soil).

For now, I have taken the one begonia out (I was advised to let its soil dry out, and then water it less frequently to see how it does). And I was hoping you guys could help me with what I should do next. I guess my question (sorry, questions) are:

1. do I isolate the other begonia that's in the same container? It seems to be doing ok right now, but I'm afraid it'll face the same fate as the one I've already taken out.

2. how can I improve the soil other than by changing it entirely? The ivy and violets still in the container are doing fine. Can I add some sand to dry it out a bit? Some perlite?

3. if the begonia does improve, can I put it back in this (new) soil?

4. if, heaven forbid, it doesn't improve and I lose them... any suggestions as to what kind of flowering plant does well in such conditions?

I'd appreciate any advice you guys could give me. I'm really enjoying how much these guys brighten up my balcony, and I'd hate to lose them.

Thanks in advance... and sorry for the huge post.