Urea actually decomposes to ammonia in the presence of water, although it only releases a small fraction of the total N. Compost tea is a good idea to help with the breakdown, I use a living humus as a top dressing in pots (only a very small amount every other month that I make myself).
As far as sterile medium goes, usually you incorporate a CRF into the mix that provides a more soluble form of N, but if you give your plants a small continuos dose of MG, then they will defiantly get enough N from the ammonia and CRF.

Wow, so really the plant uptakes it anyway. As long as the medium holds water and it is not like orchid medium that can't hold water thus the reason people do not use urea N in orchid growing. Thanks Bob!!!

Technically urea could be used in orchid type soils, but it is not ideal. Urea will release ammonia in the presence of any water, even humidity, but for large particle (>1/4") mixes it is better to use ammonium and nitrate salts.
I always nix a little nitrate in with my urea based fertilizers there is a long explanation but basically plants absorb nitrates easier than ammonium/ammonia and being able to switch between the two is helpful so that you can isolate nutrients that plants should absorb (ammonium is a cation, so if I want the plant to take up more anions like phosphate or sulfur I will make less competition and use very little nitrate; if the plant needs more cations like Ca or Mg then I will use nitrate so there is cation competition).

Hey, thanks a lot!!! I have learned quite a bit, thanks. That is interesting at larger then 1/4" is where it becomes ideal to not use urea. Nitrate is the best as it is used by the plant right away and the plant can store it as ammonium gives it instant N thus making the plant grow in a burst which can and can not be good, specially in time of fruiting and reproduction. It helps to know what type of N does what.

I found your explanation very, very informative. Thanks.

Typically, I give 4:1 nitrate to ammonium during vegitative, and 1:4 for flowering and fruit set (this is when P and other anions are crucial for most plants, and the use of the cation form allows for less competition), then 1:1 for fruit development when almost all nutrients are needed equally. For ornamentals varying these amounts means that once spring comes you can induce a larger vegitative phase, followed by a prolific flowering phase (for flowering, use more ammonium than nitrate, and also cut back on the total amount of N by about a third).

I did not know that, I was thinking it was the other way around.

Sounds like you know quite a bit about fertilizing programs, thanks so much.