How and when did the complex living system we call 'soil' evolve?
The soil hasn't always been here. At first there was probably only water and sediment. The evolution of this soil ecosystem required the transformation of rock into complex organisms within 10 million years (it sounds a lot but a mere glance in geological speak). As continents collided, the chemistry, botany and zoology came together. The plants needed the minerals and animals to colonise the land, by transforming it.
Three main aspects of the formation of soil
I make the prediction that the soil evolved around 350 mya, based on the animals found in the soil. Soil would not exist, but for the soil mesofauna. Soil does not merely “harbour” soil animals. They are part of what makes soil – as opposed to clay/minerals/sediment/sludge, a living entity.
Two continents collided to create new land. One of the properties pertinent to the 'invasion of land'/'creation of soil' appears to be the evolution of desiccation resistance. The small soil creatures like oribatid mites and springtails survive in different ways. Springtails spring out of the way. Oribatid mites roll up in a ball.
Volcanic dust provides all the essential elements for plants. It is hard to see how plants could have evolved anywhere else as there would not be that distribution of elements in say sand. If plants started to evolve in the stuff, then the little soil animals will come in and recycle those essential minerals when the plant died.