Where soil actually settles in carpet

What is actually happening

The mechanism is the pile itself — depth and surface area hold particles where a flat hard surface cannot. That is a physical property of the construction rather than a marketing position. Soil lands on the tips, works downward with every footstep, and comes to rest against the backing where the yarn is densest. It stays there until something extracts it, which is why the base of the pile is the part of a carpet almost nobody has ever seen clean. The catch is that the pile only holds so much: left long enough without extraction the load has nowhere further to go and starts riding at foot level, and sand at foot level cuts fibre on every pass. About a year of neglect is where that turns from storage into damage.

How you prevent it

Vacuum slowly and often — slow passes are what give the machine time to lift particulate up out of the pile rather than skating over it — and have the carpet extracted on a real cycle so the load leaves the building instead of accumulating in it.

Most of it drops through the pile and settles at the base, against the backing, where nothing done at the surface reaches it. A carpet can hold several pounds of dry particulate down there and still look acceptable from standing height — and that buried grit is what wears the fibre out.