When most of us think about soil, we imagine that it's simply this dark material commonly called "dirt". But soil is actually a lot more complicated than that.
That's because soil is made up of a combination of solids, liquid, & gases. That's why technically, soil isn't made exclusively of solid material like many would assume. In fact, if that was the case, we would consider it "sand" or "gravel" instead of soil.
Soil has an Upper and a Lower boundary. The Upper boundary is much easier to define, so we'll start there. It comes from the presence of air, water, plants or other live material. In other words, the Upper boundary is made of things we can see or feel ourselves, without needing any fancy equiptment.
That's what makes this boundary easy to recognize. It should be noted that if an area is permanently covered by water at a depth too deep for the growth of rooted plants, that area isn't technically considered to have soil.
As I mentioned, the Lower boundary of the soil is harder to define. That's because it's the area under the Earth's surface where all marks of biological activity ceases, usually transitioning into hard rock.
The fact that you can't see this boundary visually without the use of special equiptment explains why it's so hard to define. Below the lower boundary all that exists is essentially hard rock & Earth core materials such as magma.
What Is A Soil Horizon?
When you're talking about soil, the term "horizon" simply means "layer". In general, a horizon is the line or boundary between two things, just like the horizon line you see when you look off in the distance. Most soils have at least 3 Major horizons (A, B, C), and some have an Organic horizon. These are defined as:
A) Topsoil: Mostly minerals from parent material with organic matter incorporated. A good material for plants and other organisms to live.
B) Subsoil: Rich in minerals that leached (moved down) from the A or E horizons and accumulated here.
C) Parent Material: The deposit at Earth’s surface from which the soil developed.
O) Humus or Organic: Mostly organic matter such as decomposing leaves. The O horizon is thin in some soils, thick in others, and not present at all in others.
When put together, these horizons form something called a soil profile, which can tell scientists a ton about the life of that soil.
How Does Rock Turn Into Soil?
The process a rock goes through to become soil is called "weathering", and it's quite an extensive undertaking. In fact, this process can take over 500 years to form only 1 centimeter of soil from harder rocks.
Primarily though, the "rocks" that become soil are essentially gravel or sand that was broken down during the Ice Age that happened over 10,000 years ago. The fact that the rocks are already broken down makes the weathering process significantly quicker - but it's still longer than the average human lifespan.
What Is Soil Degradation?
It can be hard to grasp the fact that soil can become worse in quality, but it can - and that process is called soil degradation. This can happen because of improper use or bad management of the soil. Usually, this is because the soil was used for a specific purpose such as agriculture, industry, or urban development.
Put simply, soil degradation happens because of human error. That's a big problem because soil is a vital resource for all terrestrial life - which includes you, me, your favorite pet, & even that beloved tree you used to climb as a kid.