Last lesson you left a soil sorted into layers. Dead organic matter piles up at the surface, decomposition is fastest there, rain carries material downwards, and roots and burrowing animals mix as they go. The result is a soil profile, running from mostly organic at the top to mineral below.
Look closely at a profile and one band is usually paler than the layers above and below it. Nothing was added to make it pale. So why is it there?
That question is what this whole lesson runs on. A profile is not a stack of ingredients that settled into place. It is a record of things having moved, and of things having changed on the way. By the end of today you will be able to name every movement going on in a single handful of soil, and draw them.
Start by looking at what a profile actually looks like.
Source: Know Soil Know Life, Soil Science Society of America, 2012. Licensed CC BY-NC 4.0. Gallery entry
The picture labels five layers with letters. This course names four, and they are not the same four, so use the table to line them up. The middle column is what matters: every one of these layers exists because something moved.
| What you can see | Why it looks like that | Name used at HL |
|---|---|---|
| Loose litter on the surface | Dead plant material at various stages of decay. Not shown in the picture, and absent from ploughed fields, because ploughing mixes it in. | O, the organic layer |
| Dark upper layer | Mineral particles mixed with well-decomposed organic matter. Most roots and most soil life are here. | A, the mixed layer |
| Paler band below it | A layer material has been taken out of. Clay, iron and dissolved minerals have been washed downwards from it. | not named in this course |
| Brighter or redder layer | Where the material washed down from above has collected. | B, the mineral soil |
| Pale stony layer | Broken-up parent material that has not yet become soil. | C, the parent rock |
| Solid rock at the base | Bedrock, often too deep to reach in a soil pit. | not named in this course |
You do not need the letters at SL. They are here so that the picture and the course agree with each other.
Task
Three things you would see in a soil pit at Site A, the field on moraine from last lesson. For each one, suggest what movement could have produced it. You will not be able to name the processes yet, and that is the point. Come back and name them later.
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5.1.6 Transfers occur across soil horizons, into and out of soils.
Include infiltration, percolation, groundwater flow, biological mixing, aeration, erosion and leaching.
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A transfer moves matter or energy from one place to another without changing what it is. Nitrate that leaches downwards is still nitrate when it gets there. Seven of them are named in this course, and they sort neatly into three groups.
Getting water in and down. Infiltration is water entering the soil from the surface. Percolation is water moving downwards through the soil once it is in. Groundwater flow is the movement of that water through saturated rock and sediment once it has passed below the soil altogether.
Moving the soil itself. Biological mixing, also called bioturbation, is soil material moved by living things: earthworms dragging litter down, burrowing animals bringing subsoil up, roots pushing particles aside. Erosion is soil particles carried away by wind or water, which you met last lesson as an output.
Moving what is dissolved, and moving gas. Leaching is the removal of dissolved substances, carried downwards by percolating water. Aeration is the movement of air into and through the pore spaces, which is what keeps roots and soil organisms able to respire.
Notice that infiltration, percolation and leaching are three different things happening in the same downward flow of water. Water entering, water travelling, and dissolved material going with it.
Task
Water that infiltrates and percolates through the fields around Geneva does not stop at the bottom of the soil. It keeps going into the nappe du Genevois, the gravel aquifer shared across the border between the canton and Haute-Savoie. Ten wells draw from it on the Swiss side and five on the French side. It holds roughly 80 million cubic metres.
This is where leaching stops being an abstraction. Nitrate applied to a field as fertiliser dissolves, percolates below the reach of the roots, and arrives in the aquifer. Switzerland sets a limit value of 25 mg per litre of nitrate in groundwater. Nationally that limit is exceeded at around one monitoring site in six, and in areas dominated by arable and vegetable growing at more than half of them.
The Genevois aquifer itself sits far lower, at around 2.5 to 4.5 mg per litre. It is recharged mainly by water from the Arve rather than by rain percolating down through farmland, which is likely to be part of the reason. Where the water in an aquifer has come from is worth asking whenever you meet a figure like this.
Sources: Republic and Canton of Geneva; Services Industriels de Genève, water composition 2022; Federal Office for the Environment, nitrate in groundwater.
Task
Every transfer can be described two ways: by where it takes material, and by what it actually moves. Complete the table for all seven.