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.
In the context of bicycles and motorcycles, caster is more commonly referred to as head angle, rake angle or rake and trail, especially in American English. English still predominantly uses the term caster or castor angle.