Actually, caveman, bumble foot is a staph infection, not a tumor. Chickens get it from injuring a foot by scraping it or cutting it or getting a thorn or splinter in the pad. Left to fester, along with walking through poop, staph germs colonize the wound, turning into a nasty pus-filled cesspool. I'm treating a rescue hen I just adopted for a very stubborn case right now.
Madame is right in being concerned about wet, muddy spring conditions. As the weather warms up, cocci can bloom in the soil, as can botulism spores. Both can make chickens very sick.
The botulism grows in compost piles and gardens under wet, anaerobic conditions, developing a toxin that infects vegetable matter and if chickens eat this, they can become very sick. This happened in my flock last year. I didn't know at the time my compost pile was killing my chickens because I hadn't dug it and turned it properly.
You can still let your chickens free range, but you do need to keep watch for any sign of cocci or other signs of illness. Mud all by itself isn't going to hurt them, though. Chickens adore mud as much as pigs do.