Soil is the very outer layer of the earth’s crust. Its thickness depends on where you are, but generally speaking it’s the top 1.2 metres that we’re talking about – a layer of thicker subsoil iced with six inches of darker, richer topsoil.
Soil isn’t any one thing, as you can see from a handful of it. There’s a mix of materials involved, from tiny clay and silt particles, to sand and grit. Those are the main mineral ingredients of soil, but you might also find tiny deposits of various chemical elements, all useful for plant growth. Also vital are air and water, which make up a quarter of the bulk of soil each. Healthy dirt is composed of tiny chunks, and the gaps between them are important in their own right.
Alongside the sediment is the organic matter, the darker brown material known as humus. This is the remains of living things in various stages of decay – leaves and twigs, matted roots, the decomposing remnants of insects. Some of it has fallen there or blown in, some of it has passed through another creature on the way.
Then there’s all the life in the soil, the vast majority of which is tiny bacteria. There are fungi and spores, nematodes and springtails. Most obvious are the giants of the deep, those huge lumbering earthworms that can course right through the top 1.5 metres of soil, aerating it and providing channels for roots. Soil is bursting with life. There are more living creatures in a spoonful of soil than there are people alive on the earth. This is one of my favourite facts in the world.
In short, soil is complex. It’s not the background material of nature, like the flat base you build your Lego house on. It’s much more interesting than that. Some people describe soil as the ‘skin’ of the earth, and that’s a fair comparison. Our skin isn’t inert packaging to keep our insides in. It’s living and changing and is the body’s largest organ. Likewise, soil is best understood as a dynamic system, a key actor in the processes of life.
Because it is such a rich recipe, it is not easy or quick to make. Soil forms in a variety of different ways depending on the conditions, but the main one is the geological process of weathering rock. The sun, wind and rain erode stone away over the centuries, helped along by the chemical reactions of living creatures such as bacteria or lichens.
Sometimes a fresh batch of ‘parent materials’ are added to the mix by a flood or a landslide. Erosion elsewhere can bring dust or sand on the wind. Volcanic ash might fall. Fungi and bacteria work on the particles and loosen up nutrients. Other bacteria fix nitrogen. Plants grow, drawing carbon from the atmosphere and then dying back, gifting the organic matter to the earth. Worms and other creatures tunnel through, creating channels for moisture and breaking the ground up into crumbs. Slowly, patiently, the soil builds up in depth and fertility.
Under natural conditions, it can take 500 years to create an inch of topsoil. This is one of the most important facts in the world.