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.
spacer (plural spacers) ( 1 sometimes science fiction) A person who works or lives in space. 2  An object inserted to hold a space open in a row of items, e.g. beads or printed type. 3  A bushing. ( slang) A forgetful 1  person. (medicine) A type of add-on device used by an asthmatic person to increase the effectiveness of a metered-dose inhaler.