I have a really simple and maybe stupid question...
Term top soil appears very often in almost any gardening subject - but what exactly is top soil?
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Topsoil just means the top layer of soil, which might be anywhere from a five inch layer on the worse end, to a layer 12 inches thick - average is about a fork's depth, or roughly 8 inches, but it does vary from area to area.
What's beneath that is termed subsoil, and that will vary from area to area as well - for instance, throughout most of London, UK, the subsoil is heavy clay, but in other places, it may be lighter. Essentially, it will be the same mix as the top soil (in terms of silts, sands and clay) except it won't have the same level of humus material within it. This humus or organic matter in topsoil is what makes it fertile - the more humus, the more fertile, and humus comes from plant debris, debris from dead microorganisms and bacteria, or the addition of composted materials. The other difference will be the amount of life forms - there will be more in the topsoil layer than the subsoil.
Top soil is the top horizon or layer in a soil profile. It is distinguished by having the most organic matter because it is the soil beneath the organic layer; the organic layer is where the leaves and debris and insects get decomposed and then the life in the soil eats this stuff and takes it down into the top layer of soil to poop it out mixing the decomposed organic materials. This soil has the best 'tilth' for roots of plants as well as the least clay the least iron because of leaching. In no way does the darkness of this layer indicate 'fertility' but it does indicate a nice, fluffy soil or great 'tilth'.
Rarely is topsoil more than a few inches in depth. Way back in the past after the last big ice age, as the glaciers retreated back North off of the North American Continent, a huge lake was formed called Missoula Lake. The size of the Great Lakes or more. All that held it back was an ice plug and one day that disintegrated and all that water scoured the Pacific Northwest, primarily all the way down to the Columbia River. All of the top soil and life was wiped off the land. Wish I could remember the amount of water that 'lake' released but it was unimaginable.
Then the glacial till left by the glaciers, a mix of the finest soil textures all the way up to house sized boulders was further 'refined' by wind picking up the clay and finely ground rocks (flour) and depositing them in Idaho, the Palouse. They formed huge dunes. This was probably one of the 'richest' areas having the largest deposits of literally 'topsoil' in dunes 15 to 20 feet or more of just topsoil. Farmers weren't as careful as we are today about conserving the topsoil. The Palouse has less than inches now, of this incredible top soil.
Without irrigation, farmers of the Palouse, have been able to grow wheat and peas, lentils successfully. The clay particles (tiniest yet flat pieces of rock) hold onto moisture and minerals well.
I lived in Moscow, Idaho and fell in love with these undulating dunes of fantastic soil. Zone 3, with short summers but some of the fanciest, high tech, harvesters are made just for these 'dunes'. One side of the tractor can be 6' higher or lower than the other side in order to evenly harvest the side of the dune.
Hope this helps...