SOIL

Table of Contents
From Magma to Stone and Back Again

Before we can start to talk about a soil formation, we have to talk about how the building blocks for this formation come to be. And therefore we start the journey of the soil with the basics of our planet.

For our earth consists of a molten core consisting of molten rocks with a hard shell, the earth crust, that is cooled down enough so that we can live on it. This then implies that there is a temperature difference between the core and the crust. And whenever there is a temperature difference, there is a potential for movement. The hottest magma at the core will start to rise, and the cooler magma at the crust will descent to the earth’s centre. This then creates “convection” and this is the engine for soil formation. The crust gets pulled into the magma, called “slap pull”, by which volcanoes and large mountain ridges are formed while on the other end the earth crust diverges and the magma creates new land. It is an endless cycle of creation and destruction on the highest scale.

1280px-Oceanic_spreading.svg
Figure: Schematisation of the convection that creates the movement of earth plates.
Minerals

We now know how land is created and destroyed. But as soil scientists we are especially interested in the time in between. And what is especially of great importance is the building blocks of the land that is created. And these building blocks for soils are called “minerals”. Minerals are the elements build out of atoms that determine the characteristics of the rocks and stones that we encounter in the world. If the base material is made up out of Quarz for example, which consists of only SiO2, than the eventual soil that is created through the processes of erosion and sedimentation only consists of this element, implying that the soil will be very poor. But, if we have a mineral like an Amhibole, a dark mineral that consists of Ca, Na, Mg, Fe, Al and Si, the potential with regards to its nutrient status for this future soil is much higher.

minerals
Figure: Different minerals that consist of different chemical elements.

Now, in the process where the magma with the different minerals comes to the surface of the earth, there are different processes that result in the formation of stone. This formation of different rocks is called “metamorphism”. For example, we can start with Shale that then under heat and pressure is transformed into Schist, that then again is transformed into Gneiss.

meta_shale_gneiss
Figure: The metamorphism from Shale to Gneiss

Or we can start with a different rock like Limestone, that under heat and pressure is turned into the countertop in most kitchens, that is, Marble.

Marble
Figure: left Limestone, right Marble
The Destruction that Creates Life

Now we have seen how magma turns into rocks, and what rocks are made of and what they can turn into, we have come to the most important step in soil formation. Becouse what is a soil? Well,

“A soil is the first 1-2 meters of the earth surface where biological activity is possible”

Gert Peek

But how then do we go from solid rocks, to a soil where plants can grow roots, where worms can dig their pathways and where water can infiltrate to provide us with the filtered water that we need to survive? Well, here we meet the processes of Erosion and Sedimentation.

Erosion:

Erosion is the destruction of rocks into the finer elements like sand and clay. There are 3 types of erosion to be found:

  • Physical: like water erosion or weathering due to temperature changes where the water will infiltrate into a rock, then the water freezes causing it to expand which literally splits the rock in two.
frost-wedging-example3
Figure: Physical weathering of a rock.
  • Chemical weathering: for example when it rains, the CO2 in the atmosphere makes the rain slightly acidic, causing rocks like Limestone to dissolve. 
dec-31-2006-12
Figure: Chemical weathering of limestone
  • Biological weathering. this can be seen on our highways for example where plant roots are strong enough to mage their way through the tarmac and even through concrete.
6a0105371bb32c970b017c3270f324970b
Figure: Biological weathering of a rock

These are the main processes how the rock is destructed into the foundation for all life on earth. And without soils, there is no human life.

The next important process is sedimentation. It is the process where the newly formed sand is transported all over the world. Either by wind, rivers, glaciers, etc. And it is in this process that we find an eye-opening discovery.

Sedimentation: 

For we have seen that we can create smaller rocks and grin through erosion. When we go one step further in this process we find sand. And when we then include the combined effect of erosion and sedimentation we can even create particles that are so small, that we cannot even see individual particles with our bare ayes. Yet combined the form the most important aspect of fertile soils: Clay.

And it is at this microscopic scale that we find some of the most important characteristics in soils. For these clay particles are so small, that the they get a electrostatic charge due to the exposed oxide surfaces of the minerals. And it is at these surfaces that the binding of nutrients, pesticides and organic compounds take place. This is why a clay soil generally is seen as a more fertile soil than a sandy soil. 

 

Web
Figure: Kaolinite clay mineral structure

Notice again in this image the minerals like Silicon out of which the clay is built up and the Al, which in this case results in the charge of the mineral.  

The Distribution of Soils

Copyright © 2021 Lumbricus