In a pH balanced body, urine is slightly acid in the morning, (pH = 6.5 - 7.0) generally becoming more alkaline (pH = 7.5 - 8.0) by evening in healthy people primarily because no food or beverages are consumed while sleeping. Whereas, during the day the body buffers the pH of the food and beverages consumed by releasing electrolytes and the pH level goes up. This process allows the kidneys to begin the elimination process slowly.
So since you are what you eat, just drink a good amount of water and eat things that aren't too acidic, and I suppose that would be the optimum piss to fertilize your plants. haha