Humus is not just soil, it is a community of living things. Organic matter is a breeding ground for many types of microbes, including bacteria and fungi which break down plant material, and for other microorganisms that help plants roots absorb necessary nutrients.
I agree with your analysis, but I disagree that cagers refers to an entire class of persons, all motor vehicle drivers. To me, and maybe I am not using the right definition, a cager is an @sshole motor vehicle driver.
dirt bikes airborn image by MAXFX from Fotolia.com. A pit bike is a small motorcycle used for pit racing and for riding in pits; these bikes are usually used by kids while dirt bikes are motorcycles made to ride on streets and un-even roads.