Downstairs it is busy and exciting

Underground Netherlands, how exciting is it there? The geography books and wall charts from the school days taught us the types of soil with different colours: yellow for sand, light green for river clay, dark green for marine clay, purple for peat, pink for loess. The ground beneath our feet contains shiny pebbles, layers of sand, marl, peat deposits, plant and tree remains, gas and coal and many other soil substances. Every surface is different: it is hard to trudge over beach or dune sand, you sink into peat, clay soil clings to your shoes. And each type of soil has different flora and fauna, in other words: the soil determines what we see above the ground.

We often don’t know much about the underground world in which we live, work and walk. Some recent books and a documentary pay attention to that world. As it published in 2020 Underland (translated as Downworld. Traveling in deep time) by British travel writer Robert Macfarlane. In it he dives deep below the earth’s surface and describes what goes on there, including in fungal networks and caves. The Dutch documentary Below ground level, now showing in various cinemas, points the camera at everything that lives in the ground. The recently published nature diary On the skin of the heather by biologist Piet J. van den Hout gives wonderful descriptions of what is located under a specific heath area, the Regte Heide in Brabant, surrounded by stream valleys and forests. He writes of orchids working together with fungi deep in the ground and of mushrooms “rising up from the subterranean fungi kingdom.”

Earthworms and plant roots

To find out more about that unknown underworld, I set out with three people who venture into that world: geologist Jeroen Schokker, who has been working at TNO’s Geological Survey of the Netherlands for more than twenty years, and soil scientists Ingrid Lubbers and Jan Willem van Groenigen. The last two are affiliated with the University of Wageningen.

“A geologist,” says Schokker, “goes deep into the earth by drilling and other soil research. Just imagine how busy it is underground: housing construction, traffic, water management and energy transition all appeal to the foundations of our living environment.”

Van Groenigen also goes underground for his work: he conducts research into nitrogen and phosphorus, for example. Fungi play a major role in this, as do earthworms and plant roots: “Everything is connected there,” he says. “A teaspoon of healthy soil contains more organisms than there are people on Earth.”

A teaspoon of healthy soil contains more organisms than there are people on Earth

Jan Willem van Groenigen soil scientist

Both Schokker and Lubbers, who teaches soil science in Wageningen, take along an earth auger or gouge on our walk, elongated instruments with which they go into the depths and see what comes up. The walk with Schokker starts in Werkhoven in Utrecht. He has one for this area before Geological Cycling Excursion plotted from 42 kilometers. We cross a so-called bowl ground, a lower-lying area sandwiched between the Utrechtse Heuvelrug and the Kromme Rijn. As a geologist, Schokker can ‘read’ a landscape. He can “actually no longer look uninhibited,” he says. Everywhere he sees elements in the cultural landscape that have a geological origin, for example a levee, the course of a river or stream, a slope or, conversely, a depression. The underground Dutch landscape is infinitely varied.

We start just outside the village border. “Look, you can see that Werkhoven is considerably higher than the lower part of the meadow area along the Kromme Rijn, a difference of up to 3 meters. Thanks to sand brought in by the Kromme Rijn and deposited in the inner bend, habitation could arise here, as early as the ninth century. Sand was always beneficial to build on: sand gives firm, dry ground underfoot.”

Schokker is interested in how people use the landscape and what the landscape of our ancestors looked like. The fact that there is a lot of fruit growing in this area, for example, is due to a mixture of sand and clay. Sand provides a well-draining soil and clay for nutrients. Both are supplied by the river.

Soggy clay

We notice that clay hardly lets water through when we descend deeper into the bowl soil: it is wet and soggy. The water remains on the clay, even our boots lose grip on the slippery bottom. From what you observe above ground, you can determine the underground world. If a lot of water came to stand on clay, swamps with marsh plants were created. When they die, peat is formed. This allows you to reconstruct how a peat area is created: from wet clay to marsh with plants and finally peat.

Schokker calls himself Quaternary geologist, he focuses his attention on strata that were formed up to 2.5 million years ago. This period is characterized by an alternation of cold ice ages and warmer inter-ice ages. The Netherlands is first and foremost a delta, so much of what is in the ground has been brought in by rivers. “Many people think that the Alps, for example, are geologically interesting, but in our Dutch delta the partly eroded mountains are deposited as layers of gravel, sand and clay – and we find the Alps in them.” What he means to say is that the Netherlands is at least as fascinating.

Walks on different soil types

Sand from the mammoth age

At a spot right next to the Kromme Rijn, where the river meanders and we can see the sandy bank, Schokker places the drill in the ground, to a depth of about 70 centimetres. During the first drilling, river clay comes up, containing pieces of crushed red brick. This indicates former habitation. Then the drill goes deeper and fine sand comes up. Cover sand from the last ice age, twelve to thirteen thousand years old, from the time when mammoths and giant deer still roamed. Because of the cold and drought, there was hardly any vegetation here and sand was deposited by the wind. “Take the granules in your hand and rub them between your fingers,” Schokker encourages. “They shimmer and are soft to the touch. You can clearly see the individual grains, nicely rounded and even in size.” This is called ‘sorted’ in geology, he says. “This shows that the grains were transported by the wind. River sand is less nicely rounded, which feels rougher.”

The relationship between above and below ground also concerns Van Groenigen and Lubbers. Both can be seen in Below the ground level, in which they examine a pasture for soil quality. First, they explain why the original Wageningen Agricultural College is located right here on the high Wageningse Berg: three soil types come together in the immediate vicinity. The sand pushed up by land ice that made the Utrechtse Heuvelrug and the high-lying Veluwe. Clay in the Betuwe and peat in the Gelderse Vallei: the three primeval soils of the Dutch landscape.

Now I’m going to take a look at this area with them. We start on the Wageningse Berg of 42 meters high and then descend to the floodplains of the Nederrijn. Lubbers points around: “This is an impressive descent.” First down the steep slope of the moraine, which consists of sand and pebbles, then we suddenly end up in a different landscape, that of river clay. Lubbers explains: “The slope is so steep because the embankment has been cut by the river. This has to do with the course of the major rivers. From the Alps in our direction, they encountered these lateral moraines and therefore had to turn west.”

Layered landscape

We pass the abrupt transition from the bottom of the moraines to the bottom of the floodplains. This can be seen at a glance from the vegetation: old deciduous trees on the moraine and fresh green grassland in the floodplains. This is what is called a ‘layered landscape’: it was created layer by layer, always young over old. The river deposits a new layer with every flood. First sand, because the levee is still low, then more and more clay, because it is easier to transport. If the levee grows due to the heavier grains of sand, clayey particles can still pass over it.

In the film Van Groenigen also says it: without soil life there can be no life on earth. He explains: “The reason that life in the soil is so diverse is because three ‘kingdoms’ come together: soil particles, water and air. These three are so closely intertwined that there are endless different habitats for benthic life. We as soil scientists go about two meters deep, below that lies the realm of geologists. They focus on the more or less stable parent material and deeper. For us, this mother material forms the very beginning of soil formation. All this together ultimately forms our landscape. In our research, we also take into account external influences: water, drought, cold, heat. In addition, worms, microbes and tree and plant roots have an effect on the soil.”

As we walk through the floodplain, we pass a steep ridge. That is a piece of eroded levee that allows us to look at the different soil layers perfectly from the side. What at first glance appears to be just an edge of sand, actually offers an adventurous view of the subsurface, like a sample map. This is the domain of earthworms that leave plenty of traces. When you see that, you know that you are dealing with fertile soil. Just below ground level, the river clay is somewhat darker, due to the plant roots and decomposed leaves. Beneath it, the river clay is lighter in color and more sandy. Van Groenigen points to the countless small holes in the escarpment: the tunnel systems of the worms – indispensable life-bringers from the soil. We also see traces of all kinds of small creatures, such as springtails and mites. If we could zoom in, we could even see algae, rotifers, nematodes and millions of microscopic organisms.

Surprise lifted

Soil scientists also first look at what is above ground and then reconstruct the invisible subsurface. Further downstream, the chimney of a former brickworks reaches upwards. “Why is it right there?” Lubbers asks aloud. I suspect because of the clay soil, being so close to the river. Almost correct. Close to the river means a levee of sand. That is again mixed with clay. Sandy clay is the best raw material for brick.

You never stop learning about the soil under your feet, she argues. We are still going to the Gelderse Vallei, a peat area northwest of Wageningen. Lubbers does not need an earth auger here, but uses a gouge, a long iron bar in the shape of a gutter. When you turn this, you bring up soil. She easily pushes the instrument a meter into the peat. It is soaking wet, the water is level with the ground. That’s doom; rising water that fell on the Veluwe thousands of years ago and rises here. The result surprises her: the soil profile indicates peat on clay, while in most places there is cover sand under the peat: “This is therefore very heavy clay under the peat, this is called bowl clay, deposited far away from the river. Another proof of how variable the subsurface can be.”

Second surprise: in the reedy peat we find an orange-coloured piece of alder wood – which could be six thousand years old, Lubbers notes.

In the past dozens of boreholes were sometimes drilled per square kilometre, sometimes only at a distance of a few meters from each other, precisely because the subsoil in a country like the Netherlands has such enormous variation due to the great forces of ice ages, rivers and wind. That also explains why geological maps and soil maps have so many different colors: a different one for each layer and each soil type.

The ground under Dutch feet as a fascinating, permanent voyage of discovery.