Type of trail, condition and location: its effects on mountain bike tire pressure

I rode Brunswick Nature Park on Sunday. It was wet and the type of dirt they have, which is sand, was feeling mushy. I was going at a good pace but nothing close to race pace. At those speeds it took me about a couple of turns to figure out my rear tire did not have the right PSI. I usually run 28-30 psi on the rear. I knew If I took a turn aggressively I would have probably burped the tire. There was nothing scientific about this until I guessed the pressure on the tire. I said to my friend, "The tire probably has about 25 psi" and seconds later I was able to verify it with a pump. Moral of the story, it pays to know your psi.