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Take the soil in my mother's backyard, for example.After about 18 inches the soil grades into a two-foot matrix of solid, smooth clay mixed with boulders.) Just because a patch of topsoil takes x centuries to build up doesn't mean that the land is x centuries old.Most likely, that topsoil began to build up only recently, geologically speaking, and has either reached a practical limit to its depth or has been subject to erosion.Geologically speaking, any given patch of land is seldom in equilibrium for long.Either it is collecting sediment or being eroded away, usually the latter. Water-borne sediment will be washed in from higher ground, perhaps hills and mountains hundreds of miles away.I suspect that most of them belong to plants which were chopped down years ago.There's not much down there in that clay to completely rot them away.

In the case of erosion, the topsoil, of course, is removed.Forget about billions of years of soil accumulation!Where sediment is neither being collected nor eroded, soils necessarily take their mineral components from the underlying parent rock.Consequently, there is a practical limit to how deep the soil can get even if erosion never occurs.The accumulating humus will also reach an equilibrium, when new material balances that lost by decay and oxidation.

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