When urine dries it becomes and turns alkaline.
Urine left out in the air will eventually turn to ammonia which if contact with skin can lead to a burning rash.
For example if you were to pee your pants and not change for hours then the pee that is drying on your pants and against your skin will begin to smell.
Then eventually the urine will cause rashes on your skin.
I've experienced that as a kid when I peed my pants when I was 7 years old.
I was embarrassed so I waited outside until it dried but my mom found out anyway as the peed pants smelt and I smelt of old urine.
I then got a rash afterwards.
When urine is first deposited onto a floor or fabric, it has a pH of about 5 or 6, which is on the acid side of the pH Scale. It is easier to remove right then when it is fresh. Once it dries it turns “alkaline” or to a high pH between 10 to12 on the scale and becomes more difficult to remove.
When you're dehydrated and your pee gets very concentrated, it can smell strongly of ammonia.
If you catch a whiff of something really strong before you flush, it might also be a sign of a UTI, diabetes, a bladder infection, or metabolic diseases.
When stored for long periods of time, urea decays into ammonia.
Ammonia in water acts as a caustic but weak base.
The reuse of human urine is specifically receiving increasing attention as an alternative fertilizer because it contains most of the nutrients of human excreta (9) and it can yield considerable amounts of nitrogen (N), phosphorus (P), potassium (K), sulfur (S), calcium (Ca), and magnesium (Mg).
Urine is non-toxic, although it contains urea and other substances which can be toxic if they are not excreted and reach high concentrations in the body.
Incidentally, something is either sterile or not.
If urine is kept for a long time at room temperature, it will give an ammonia smell produced by the bacteria, which will decompose the urea in the urine.
pH: Growth of the proteus will change the pH into alkaline.