Since when are there soils?

One answer is: as long as there have been terrestrial surfaces

An impression of early soils, made in leonardo.ai. Since when are there soils?

A simple question, but like often with such questions, the answer is perhaps a bit more complicated.

Soils are bodies that occur at the intersection of four spheres, having once been described as the most complex biomaterials on Earth: the geosphere, the hydrosphere, the atmosphere and the biosphere. The latter point means: no soil without life. So in order to answer the question, we could start by asking since when has there been a terrestrial surface that had life.

It turns out there very likely has been terrestrial (i.e. non-marine) life as soon as continents emerged, with some reports putting first evidence at around 3.2 billion years. This was microbial life, of course, which first arose in the ocean some 3,8 billion years ago. It seems likely that endolithic (living inside rock) microbes were the starting point for soils; they would have been a bit more protected. The question is then, given that there has been rock material, water and an atmosphere, and the presence of life — does this mean there are soils as well?

I would answer this question with ‘yes’. But there can be other answers, depending on how demanding you are with the definition. Some others call these protosoils. Weathering is often an important ingredient in soil formation, so a modification of the rocky mineral material. But I cannot imagine mats of microbes growing on these early land surfaces without also affecting the rock they grow on in some way. At the very least they will have deposited organic material, necromass, another important ingredient of soils. Is there a minimal amount of carbon in the substrate before you call something a soil?

There certainly is a very strong bias to regard soils as inextricably linked to the presence of plants. And this is certainly mostly the case now, and soil science definitions reflect this fact. But terrestrial ecosystems will have started without plants and their roots.

No doubt the arrival of land plants, and then later also plants with roots, was a game changer in the Devonian. A direct carbon pump for the soil, so to speak, with roots injecting carbon deep into the substrate. This happened quite a bit later though, so around 500 million years ago. This means billions of years of potential soil without plants, if you subscribe to this notion. That is quite interesting, isn’t it? We think of terrestrial ecosystems as plant-dominated, but likely for the longest time they weren’t.

What does this mean? Maybe that there is quite a bit of microbial life in soil that is very independent of plants. Arguably that is the case. Clearly in some of the extreme environments on Earth now, where there are no plants. Or the deeper we go into the soil, the more microbial life will be independent of plant inputs and environmental modification. Are we traveling back in time when we go deeper down into the subsoil?

It could also mean that the dominance of plants is just a visual effect: when we look at the soil, we mostly see plants covering it. We also understand soil mostly in terms of plant effects on their substrate; and we study feedbacks between plants and the soil. But perhaps the real nucleus of the terrestrial ecosystem is the soil. Maybe the soil is in the driver’s seat. Perhaps the soil is what persists. Just at some point it gave itself plants.

Hope you enjoyed these musings. If you know of any literature that delves into this, please share it in the comments!