Earthworms and other beneficial soil organisms like sowbugs, maggots, beetles, etc., will do the most work far as mixing and putting nutrients and organic matter further down into the topsoil. Over time, 12-24" could be effected by natural forces.
Also keep in kind that powerful liquid biostimulants like aerated compost or seaweed tea or dry molasses mixed in stuff like fish emulsion, etc., will pull nutrients and beneficial microbes down further into the topsoil over time too.
Since I'm been doing no-till gardening for the last 7 years, I totally depend on the earthworms and microbes to mix and blend and pull the nutrients and organic matter further into my topsoil every year. The more compost and liquid biostimulants you use, the greater the potential to get deeper and richer topsoil over time.
Happy Gardening!
_________________ The entire Kingdom of God can be totally explained as an Organic Garden (Mark 4:26)
William Cureton
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