Phase 1: Twenty-four thousand years ago, you were under ice

Start with the ice

Open the federal map viewer and look at where you are standing: Switzerland at the Last Glacial Maximum.

https://s.geo.admin.ch/zw89c8w6a1tm

This is swisstopo's map of Switzerland at the Last Glacial Maximum, the coldest point of the last ice age, around 24,000 years ago. Find Geneva. Find the lake. Both are buried under the Rhône glacier, a tongue of ice that filled the entire basin and reached as far as the Lyon area, well off the western edge of this map.

The ice did not finally leave this region until roughly 16,000 years ago. Everything you can see from the classroom window, every field, every vineyard, every garden, has been building its soil since then and not one day longer.

Now put a number on that. The Federal Office for the Environment estimates that in Switzerland it takes on average about 100 years to form one centimetre of soil.

Task

The ice left this region about 16,000 years ago. Calculate the greatest thickness of soil that could have formed here since then, using a rate of 1 cm per 100 years.
Suggest what your answer to question 1 means for a farmer who loses 2 cm of topsoil from a field in a single storm.

Here is what all of that has left beneath the school. The soil is only the thin part near the top. Everything below it is what the soil was made from.

Soil is a system

5.1.1 Soil is a dynamic system within the larger ecosystem that has its own inputs, outputs, storages and flows.

Soils are a resource for life and they vary widely.

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You already have the tools for this. A system has things entering it, things leaving it, things held inside it, and movement between the parts. Soil is no different.

Inputs are what enters the soil: dead leaves, weathered rock, rain, manure, fertiliser, sunlight.
Storages are what the soil holds: mineral particles, organic matter, water, air, living organisms, plant nutrients.
Outputs are what leaves: gases, heat, dissolved nutrients washed downwards, particles carried away by wind and water, minerals taken up by roots.
Flows are the movements between all of these.

Soil is an open system, because both matter and energy cross its boundary. It is also dynamic. It is not a finished object but a set of processes still running, right now, under your feet.

The guidance adds three words worth holding on to: soils vary widely. Two soils a short drive apart can behave completely differently, and the rest of this lesson is about why.

Task

Outline what is meant by describing soil as a dynamic system.
Explain why soil is described as an open system.

Phase 2: What soil is actually made of

5.1.2 Soil is made up of inorganic and organic components, water and air.

Soil is a complex mixture of interacting components forming its own ecosystem with distinct soil organisms. Inorganic components or mineral matter (rock fragments, sand, silt and clay) come from weathering of parental rock. Organic components include living organisms and material from the decay of organisms. There are keys published online to classify soils.

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Four things in one handful

Pick up a handful of soil and you are holding four things at once.

Inorganic components. The mineral fraction: rock fragments, sand, silt and clay. This is most of the mass of most soils. It is produced by weathering, the breaking down of the parent rock beneath, sometimes called parental rock, or of rock material that something else dumped there.

Organic components. Everything that came from living things. This splits in two: the organisms actually living in the soil, such as earthworms, insects, fungi, bacteria and roots, and the dead organic matter they feed on. Broken down far enough, dead organic matter becomes humus, the dark crumbly material in the upper layers.

Water. Held in the spaces between the particles, carrying dissolved nutrients with it.

Air. In those same spaces. Water and air compete for the pore space, which is why a waterlogged soil holds almost no air and why drowned roots die.

These four do not simply sit together. They interact, and the result is an ecosystem in its own right with its own community of organisms.

Task

Identify the four components of soil, and distinguish between the living and the dead parts of the organic component.
Outline where the inorganic component of a soil comes from.
Explain why the proportions of water and air in a soil cannot both increase at the same time.

Local example: two soils, two hours apart

The Geneva basin: soil on moraine.

When the Rhône glacier melted back it dropped everything it had been carrying: clay, silt, sand, gravel and boulders in one jumbled mass, dumped without sorting. That material is moraine, and it is what most soil in this canton sits on. The glacier stalled and readvanced several times, so ridges of moraine were piled on top of the ground moraine left underneath the ice.

Two things about moraine matter here. It is unsorted, with particles of every size mixed together, which is why fields around Geneva keep turning up stones. And it starts out lime-rich: cantonal soil surveys put the limestone content of Rhône glacier moraine at around 25% before weathering, carried down from the Prealps. As soil formed, that limestone was slowly dissolved and washed out, leaving the tougher silica-rich rock behind.

The Rhône plain in Valais: soil on alluvium.

Upstream of the lake, between Martigny and Sion, the flat valley floor is built of alluvium: sediment carried by the river and dropped wherever the water slowed. Rivers sort what they carry, so alluvium is laid down in layers, coarse where the flow was fast and fine where it was slow.

Until the nineteenth century this was a braided river that shifted its channels and flooded, burying its own deposits over and over. The first Rhône correction (1863 to 1894) and the second (1930 to 1960) straightened, embanked and drained it, and the plain became some of the most intensively farmed land in Switzerland. A third correction was decided on by the Valais Grand Council in September 2000 and is still under way, although the cantonal government ordered a full revision of the project in 2024.

https://map.geo.admin.ch/#/map?center=2609564.98,1167194.3&z=2.167&lang=en&topic=ech&layers=ch.swisstopo.geologie-geocover,,1&bgLayer=ch.swisstopo.pixelkarte-farbe

Phase 1: Twenty-four thousand years ago, you were under ice

Start with the ice

The ice left this region about 16,000 years ago. Calculate the greatest thickness of soil that could have formed here since then, using a rate of 1 cm per 100 years.

Suggest what your answer to question 1 means for a farmer who loses 2 cm of topsoil from a field in a single storm.

Soil is a system

Outline what is meant by describing soil as a dynamic system.

Explain why soil is described as an open system.

Phase 2: What soil is actually made of

Four things in one handful

Identify the four components of soil, and distinguish between the living and the dead parts of the organic component.

Outline where the inorganic component of a soil comes from.

Explain why the proportions of water and air in a soil cannot both increase at the same time.

Local example: two soils, two hours apart

Layers, and the time they take