When did the soil evolve? 
If soil is a living entity, like most of us consider it is, then it must have evolved. Although I have heard version that is was dropped on us from a great height. In which case, when? It hasn't always been there. When do you think it is? As you unravel that question, the next that pops up, is 'How did it evolve?'

Here is my (first) version of an answer to those questions.

Many people might think that 'when did soil evolve? an odd question. We don’t hear many people discussing when and how the entity/body/biome/ecosystem called ‘soil’ evolved. It is taken as a given. Yet it must have evolved - from something else - but what? If the soil is ‘living’ then the various organisms must have co-evolved. Yet it fails to get a mention. It is as if it has always been just there. Under our feet. But it hasn’t. The first life was probably in water or sediment, which is a long way from the composite, friable, clod of life we know today as ‘soil’. Yet, we would not be where we are today without it. We talk about the evolution of plants and animals, but rarely hear about the evolution of soil. Yet soil is pretty well the same the world over – which tells us something.

We refer to the soil as ‘the living soil’, made famous by the self titled book by Lady Eve Balfour. We keep forgetting it is alive and a whole complicated ecosystem exists of countless relations not just between the fungi and worms, a popular perception. All the 100s of species mesofauna – small but quite minute creatures consisting mainly of mites, springtails, nematodes, live alongside the massive earthworms. There are many species, eating spores, roots, other creatures and providing the massive surface areas for the microflora to work on.

The vital part of the soil – which literally binds the world together, is only about an inch thick. It should not be seen as just a receptacle for chemical inputs. Soil is not just a physical or chemical entity but a biological ecosystem. An ecosystem that is capable of transforming rock into complex organisms within 50-100,000 years. There have been places and times when the fragile structures have broken down leading to dust bowls. Without it, quite simply, we along with many other creatures, are dead. Plants tend to be seen as an addition to the soil, yet it looks more likely that they developed relationships with soil animals - springtails and mites, integral to the evolution of soil. 

These animals are not just an after-thought thrown in after the usual physical explanation for the development of soils where plants and later animals are thrown into the equation with no real analysis of how those biological entities transform the physical and chemical organisation. What is their role? We make guesses based on the animal appearance eg we can make out that some mites are predators not herbivores, by the shape of their mandibles. Darwin taught us about how earthworms munch and crunch their way through. We now need somebody to explain the relationship of the worms to these other  fragile living soil structure, and how these co-evolved and now co-exist. So if this is living – perhaps not an organism, but certainly something like Gaia predicts.

So when do you think soil first appeared?

In the beginning was the Big Bang

Among the trillions of suns created,

In an obscure galaxy, called the Milky Way

A planet flew from one of the stars, our sun, about 4.5 billion years ago

Consisting of rock, sand and water, 

Life appeared about 3.5 billion years ago (bya),

Starting as cells in sludge

Simple animals/plants appeared about a billion years ago

So the living soil evolved sometime after that 

Probably around 350 million years ago

A tenth of the time life has been on 'The Earth'

Yet that is what we call our planet now.

From having counted a half a million of the small moving creatures, I knew that most of the creatures I counted were - taxonomicaly speaking, 'primitive'. That means they were around before insects and all the later terrestrial creatures like dinosaurs. The main animal inhabitants of soil are springtails, mites nematodes and earthworms. All primitive, first appearing 400 mya. They were in the land before insects arrived. They created the soil food web, along with the fungi and bacteria. It is this soil that gives the planet its name - Earth. It was this that  enabled all those 'Higher' plants and animals to appear. That we now wonder at and call 'Nature'. Yet we treat this soil like dirt.

My Theory
Ahem. My theory, my theory is that soil evolved between 350-375 million years ago (mya). This was originally based on how soil animals aided evolution of both soil and plants, in a three way relationship that was to create the earth as we know it today. See...
Role of Soil Animals in Soil Evolution, and Role of Soil Animals in  Plant Evolution. 

Then someone said:"Has your theory about birth of the earth got anything to do with volcanic ash?  Because there was a lot of it around the same time 375-350 mya. Looking for the answer to that led me to this: 

Essential Elements and the Evolution of Higher Plants 
These seem to firm up many of the previous 'theory'., showing the roles of soil animals in relation to both soil and plant evolution.  

As the two continents collided to make one, there was much volcanic activity and the resulting ash gave a perfect substrate for creatures to come on to the land from out of the sea and develop higher forms of plants and the soil itself. And the role of soil animals was probably crucial in stopping all the volcanic ash and plant debris just going into the oceans; instead the essential elements were recycled. In the soil..
So while it is a curious way to look at the history of the world - from the perspective of soil animals, it does allow us to bring in several other different sciences, and enables us to look at the whole better, and the movement and mechanisms of change become clearer.

Start the story with Role of soil animals