Nitrogen in Urea is very available.
TOO available really for use as fertilizer unless you REALLY know what you are doing... and even then, only a very few people would recommend using piss as fertilizer... I'm NOT one of them.
You want high N without the risk of burning your plants and having to piss in them (why oh why does this keep coming up, don't pee in your plants please!), use fish emulsion.