Topsoil is primarily mineral, ie ground up rock, with about 5% organic matter. Material from a compost heap is primarily organic with little or no mineral material. Compost and soil are two completely different things though are often mixed together. Soil forms as the bed rock is broken down by weathering. Speed of breakdown depends on many factors.
I can tell when I get down to my subsoil - it's like red builders' sand with pebbles in it. The topsoil depth varies depending on (I think) how much a particular area has been cultivated, mulched etc over the years. When I took up some lawn a few years ago to make the border bigger, there was probably only 4 inches or so of topsoil - the grass would have most likely been sown (or turf laid) in about 1950, shortly after the house was built, I guess with no soil improvement first.
Doncaster, South Yorkshire. Soil type: sandy, well-drained
Just looked up the rate of peat bog deposition and that is approximated at 1mm per year so could get to a depth of 50cm in that time frame, so looking at a soil where the organic material is not preserved by the waterlogged conditions in a bog, 20cm seems like a reasonable figure for the development of a layer with organic material and lots of living things.
What type of plant life is growing on the soil will have a big impact in my opinion with deep rooted trees and shrubs leading to deeper topsoil while smaller plants with shallow roots leading to it developing a more limited layer with organic material in sufficient quantity to count as topsoil. If there was no plant cover an area could actually be loosing topsoil over that time. Silt from topsoil eroded on high ground could also lead to a much faster rate of topsoil developing where that material is deposited like in the lower areas of river valleys. I think the statement could be true in some places but likely to be more or less in many others.
Topsoil is primarily mineral, ie ground up rock, with about 5% organic matter. Material from a compost heap is primarily organic with little or no mineral material. Compost and soil are two completely different things though are often mixed together. Soil forms as the bed rock is broken down by weathering. Speed of breakdown depends on many factors.
Thinking about exposed rocks being worn down into tiny particles, then washed away and deposited, 500 years seems rather a short time to build up 20cm. Maybe it's mostly in the beds of fast-flowing streams where stones get ground against each other, rather than wind and rain. Any geologists on here?
Doncaster, South Yorkshire. Soil type: sandy, well-drained
Any such statement is really meaningless. There are so many variables involved. It would presumably take far longer in the Artic than in temperate climates for example.
Your original quote says "it can take 500 years to form just 20cm of topsoil"
I doesn't says it always does, just that it can. I presume the point is that mineralised top soil is precious and very easily washed away or otherwise eroded. It is not usually held to be an important resource. Once it's gone, it's gone. Like the ancient peat bogs.
" By some estimates, it can take 500 years for healthy topsoil to develop and less than a century to degrade."
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Material from a compost heap is primarily organic with little or no mineral material. Compost and soil are two completely different things though are often mixed together.
Soil forms as the bed rock is broken down by weathering. Speed of breakdown depends on many factors.
Edited to add:
https://horticulture.co.uk/compost-vs-soil/
See that laughing face there at the end of my sentence? That means i'm not being totally serious and having a bit of a joke
"Have nothing in your garden that you don't know to be useful, or believe to be beautiful."
Such statements are best ignored.
as a non-renewable resource as it can
take more than 500 years to form 2cm of
topsoil.