The universe below our feet

As children, we used to love playing in it. We would carry it on our knees and under our fingernails. When we would enter the house for dinner, our parents would make us wash it off our hands before eating. The dirt made us dirty, and only clean kids were allowed at the dinner table. Little did our parents know the true value of the dirt under our fingernails. Little did we know that one teaspoon of soil contains more organisms than all human beings who have ever lived. And that this life below the ground remarkably resembles the life in our own guts. Let’s take a trip to the universe below and have a closer look at our inextricable connection to dirt.

The layers of soil

The universe beneath our feet is built up over a vast time span. The very first soils as we know them now appeared roughly 450 million years ago, formed by a dynamic and never-ending weathering process. The soil’s original mineral or rocky parent material broke down under the whims of time, climate, and living organisms. Over time, it transformed into a mixture of minerals, gasses, water, organic matter, and billions of living organisms. What’s left today is a complex and interrelated system of dead and living, organic and inorganic material. A few centimetres to several meters below the surface, we can still find the original rock or minerals: the bedrock. It’s carrying the outcome of the soil formation process on its solid shoulders.

Directly above the bedrock, we find the C-Horizon, a partially weathered and broken-down layer of parent material, in which only the deepest, strongest and most determined plant roots and soil microorganisms survive. Moving up, we enter the next layer that we call the subsoil. Functioning as a storage centre, this is where we find a collection of washed-down minerals and nutrients. Getting closer to the surface, we find the topsoil. Home to an enormous population of organisms, bacteria, fungi, plant roots, decomposed plants or animals, nutrients, sugars, water, and carbon. Then finally, squinting our eyes against the sunlight, we end up in the top layer. One that is like icing on the cake: decomposing plants and animals, such as the leaves of a tree in autumn or a fallen apple slowly rotting away.

Given the complexity of the process of soil formation, it’s not surprising that it can take hundreds of years to build up a mere 5 millimetres of healthy topsoil. And all that hard work can be undone in a matter of hours by a single landslide, volcanic eruption, bulldozer, or plough.

The rich economy in the soil

 The biggest population in the topsoil is what we call ‘the microbiome’; a community of fungi, bacteria, viruses, eukaryotes and archaea. They collaborate in breaking down dead matter and turning it into new life. The bacteria, who make up the largest part of the microbiome, are the earth’s champions in decomposing organic matter and releasing nutrients for uptake by the plant’s roots. Plants soak up sunlight and CO2 through their leaves. They transport sugars and carbon down to the soil. Down in the soil, the plant roots trade carbon for nutrients with fungi and bacteria. The fungi protect roots from different plants and trees from diseases. In addition, the enormous network of fungal threads also enables trees to communicate with each other. As Koen describes, “when you speak with soil specialists, carbon is almost like a currency. It is in a constant flow between plants, roots and organisms: from up to down and back.”

The economy in our gut

What about our own bodies? The Human Microbiome Project discovered that the microbiome in the soil and in our gut contains a similar staggering number of active microorganisms. Just like the soil, our bodies are home to a symbiotic economy of bacteria and fungi. Evidently, the microbiome keeps the host healthy and thriving. It turns our food into nutrients and vitamins and helps our immune system fight diseases.

Just like in healthy soil, our body holds and trades carbon – which we get by consuming plants or animals. The carbon in our body makes up 16% of our body mass. It fuels our microbiome’s power plants and provides the building blocks for the stuff of which we are made. When burning it, we take in oxygen and breath out the waste-product carbon dioxide. In turn, plants breathe this in, with all the help they can get from the soil in which they are rooted. In turn, they produce carbohydrates out of carbon dioxide while releasing the oxygen that we breathe. A perfect circle of interdependency.

We are more intimately connected to the soil than our parents imagined when they made us wash our hands before dinner. We trade, breathe, and thrive symbiotically.

SO, LET’S EAT SOME DIRT?