Oh my god! What the hell happened?
Make sure you clean that out good -- use some triple antibiotic AND antibacterial soap! There are all sorts of weird spores and bacteria here in the desert, and it could get infected with all sorts of weirdness. If you notice any redness radiating from your knee, get to a doctor immediately.
(This girl I knew was playing soccer somewhere in town and took a tumble. She didn't wash it right away because she didn't want it to bleed and get her pulled from the game. Within 72 hours, she had some kind of infection that was spreading to her heart and had to be hospitalized. It was more than two weeks before she was back to normal!)