warm water and nitrates . . . but not immediately. The nitrogen in urine is mostly urea (NH2CONH2) and ammonia (NH4+) and so not easily or directly usable by your plants. They much prefer to obtain their nitrogen as nitrates (NO3-), so depend on the microbes in the soil to do the conversion. This is done in a two-step process:
1) ammonia to nitrite: carried out by Nitrosomonas
2) nitrite to nitrate: carried out by Nitrobacter.
So that's a reason why it's not efficient to pee straight on your vegetables, the intermediates (and some time) are necessary. Back of an envelope calculations and a bit of scoping out the internet suggest a) that the average adult voids 1.2 lt (make that 3 yankee pints) a day, mostly water but including about 10g of nitrogen. This is enough, if spread about judiciously and through the year, to provide 50-100% of the nitrogen requirements for plants sufficient to feed an adult. So that's kind of neat in a self-contained way. But a 1% solution of this stuff is muy potente, not least because urine is also rich in salt, so it's better to cut it 1:10 with water. I'm glad I found that all out, and therefore I'm glad I got to teach The Nitrogen Cycle for the Environmental Science Module at The Institute