The volume of topsoil when you multiply it out at x feet deep by y millions of acres is truly staggering, but you can certainly build rich soil very quickly (si

The volume of topsoil when you multiply it out at x feet deep by y millions of acres is truly staggering, but you can certainly build rich soil very quickly (single digit years) if you only need a small farms-worth. Occasional grazing and fire is not the most efficient method. I can't help but recommend the book "farmers of forty centuries" by FH King for a look at what it takes to produce 3 seasons of crops per year to feed the populations of China/Japan/Korea on very slim, mountainous real estate circa 1900. Spoiler alert: it's a lot of hu-manure and manual labor.