Pete, I've never known urea to be harmful to the plants so much as useless. It needs to be broken down by soil microbes before its nitrogen can be converted into usable form, and since orchids aren't (typically) grown in soil, the urea never gets broken down.

Here, we use fish emulsion and seaweed extract regularly, with results that I'm really happy with. I mix a batch of it up to last a couple of weeks, and the plants love it--it doesn't take much, either.

As far as lowering the pH, plain old vinegar will do a great job.