ESR: Lesson - Soil Formation (Topic 4.2) 📖

⏳ Estimated Reading Time: 9 - 11 minutes

Describe the characteristics and formation of soil.

Soil Formation

soil with tiny plants sticking out — **Soil has both living and nonliving components. Soil is not just "dirt."**

Soil is formed by the weathering of rocks but also has biological components as well, giving it the ability to support plant growth. Land plants are the basis of the terrestrial food chain; thus, soil forms the basis of terrestrial ecosystems. Soil is made up of mineral particles, air, water, and organic matter.

The bulk of the organic matter in soil is humus, highly nutritious organic matter that helps support plant life. Nutrients from decaying organisms are released into the soil and absorbed by producers (plants), allowing the nutrients in the decaying organisms to be recycled.

Soil is considered an ecosystem because it has both biotic (living) and abiotic (nonliving) components.

Before microbes evolved on Earth, there was no soil, because soil requires microbes. About 500 million years ago, true soils began to appear. Microbes used the chemicals in the rocks to build their bodies and when they died, their organic matter was added to the first soils.

Once the first simple soils developed, land plants could develop, which allowed more soil to form through weathering and dead organic plant material. This allowed soils to become more complex and nutritious, creating hospitable habitats for early land animals. These early land animals helped mix the soil by churning it and adding their own organic material to the soil when they died.

Now, microbes and fungi help most land plants access the rich minerals and nutrients in the soil.

Weathering

When rocks are broken down, it is known as weathering. There are 3 main types of weathering: physical, chemical, and biological.

Explore the presentation below. Have you seen any of these types of weathering before?

Essential Knowledge

Soils are formed when parent material is weathered, transported, and deposited.

Soil Horizons

Soil is organized into vertical layers known as horizons. Soil horizons are layers of soil that have different physical, chemical, and biological properties. They are formed by the processes of weathering, deposition, and decomposition of rocks and organic matter.

A vertical slice of soil (similar to a slice of cake) is known as a soil profile. The lower you go on the soil profile, the less organic material you will find.

Explore the soil profile below to learn more about soil horizons. Have you noticed different soil layers while digging in your garden?

Just tap on the white dots at the bottom to uncover all the fascinating details!

Essential Knowledge

Soils are generally categorized by horizons based on their composition and organic material.

Erosion

Erosion is the process of soil and humus particles being picked up and carried away by water or wind. When a soil has good infiltration, runoff is minimal and, therefore, erosion is minimal. Lighter particles (clay and humus) are the first to be carried away. This is important because humus contains the bulk of the nutrients in the soil and is critical to plant growth.

Explore the tabs below to learn more about each type of erosion. Can you see evidence of any of these types of erosion around your house?

Water
Wind
Glacial

Water Erosion

Water erosion is like the Earth's way of giving the soil a little makeover — but instead of a gentle spa treatment, it's more like a vigorous scrub! When it rains, the water can wash away bits of soil in different ways. Sometimes, the raindrops make a big splash, sending tiny bits of the soil flying. Other times, the water flows gently over the surface, creating a thin sheet that slowly carries little soil pieces along. But when the rain gets really intense, it carves out tiny channels called rills or even larger ones known as gullies. It's nature's way of reshaping the land, one raindrop at a time.

Just imagine if the soil could talk — it might say, "Whoa, that was one intense shower!" Water erosion is a powerful sculptor, shaping the landscape over time.

Explore the interactive below to learn more about the different types of water erosion. Have you seen this type of erosion at your house? Before we put gutters on our house, we had significant splash erosion where the water from our roof ran off into our flower beds.

Wind Erosion

sand dunes — **Wind moves sand and forms dunes on the beach.**

Wind is not just a gentle breeze; it can also be a powerful force that shapes the Earth's surface. In dry areas, wind can carry sediment and use it to erode rocks over time. Dry soils are particularly vulnerable to wind erosion because they lack the moisture that helps hold soil particles together, unlike wet soils that are more stable.

So next time you feel the wind blowing, remember that it has the potential to shape the very rocks beneath your feet!

But be careful, wind-blown sand and soil can also be abrasive, similar to the feeling of rubbing sandpaper against your skin. Now, picture this abrasive force acting on rocks or mountains over thousands of years.

That's right, wind erosion can slowly wear down even the toughest rocks, showing us the remarkable impact wind can have on shaping our planet's landscape.

Glacial Erosion

retreating glacier forming a valley — ***Retreating glaciers can carve valleys out of mountain ranges, as seen in this image.***

Glaciers, massive formations of ice, can be found in regions where the temperatures remain cold enough for year-round ice. These icy giants, covering approximately 10% of the Earth's surface, exhibit a fascinating ability to flow steadily downhill, shaping the landscape as they move. It's hard to imagine, but during the last ice age, glaciers were even more widespread, engulfing nearly 30% of the planet's surface in a frozen embrace.

Not just stagnant blocks of ice, glaciers are powerful forces of nature that can surprise us with their capabilities. As they slowly traverse the land, glaciers have the incredible strength to transport large boulders and sculpt the terrain, shaping valleys and carving mountains over extended periods of time.

Essential Knowledge

Soils can be eroded by winds or water.

Effects of Erosion

hard, dry sediment — **Desert pavement lacks humus and topsoil, hindering plant growth.**

Erosion is an example of a positive feedback loop because it can turn into a vicious cycle if it is not controlled. In the most extreme cases, soils can be completely destroyed, leaving only the heaviest sand particles.

When this happens, the result is considered a desert pavement, because all that is left is a thin surface layer of stones and gravel.

This protects the soil underneath from further erosion, but the most nutrient-rich top layers are already gone, greatly impeding plant growth.

Now, take some time to explore the effects of each type of erosion. We will discuss the consequences of these effects in future modules.

Water
Wind
Glacial

Effects of Water Erosion

An image of sediment flowing into the Beaufort Sea, causing considerable cloudiness in the coastal waters. — ***Eroded sediment flows to the ocean via waterways. Heavier sediment settles near the river, lighter sediment farther at sea.***

Water erosion is a natural process, but it can be accelerated by human activities, such as deforestation, overgrazing, improper irrigation, and urbanization.

Water erosion can pollute waterways. When the eroded sediments flow into waterways, they can bring pollutants with them. The sediments themselves can also pollute the waterways, filling critical habitats with sediment.

Water erosion that is left unchecked can cause a loss of nutrient-rich topsoil and humus, which reduces soil fertility. This can cause a reduction in crop productivity on agricultural lands and biodiversity in natural areas.

Water erosion can also form canyons, valleys, and river deltas, which can create habitats for different species.

The Grand Canyon was formed from water erosion by the Colorado River over millions of years. This erosion exposed some of the oldest rocks on Earth and created habitats for the organisms living in the canyon.

Effects of Wind Erosion

The great plains with tall grasses blowing and sand is blowing in heavy wind. — ***Wind can blow freely across the great plains. This can cause problems when fields do not have plants to hold the soil in place.***

Wind erosion can create sand dunes, which protect coastal areas from further erosion. If you've been to a beach with dunes, you've probably seen a sign prohibiting you from walking on the dunes. This is to protect the beach and dune grasses.

Wind erosion can also pollute air, water, and land with dust. This can cause respiratory problems in humans, affect visibility, and climate. For example, it is thought that the meteor that hit the Earth during the time of the dinosaurs created so much dust and debris, that the sun was blocked and Earth's climate cooled.

Blowing sediment can also damage property and infrastructure by sandblasting or by simply burying them.

Wind erosion can also reduce soil fertility by removing the nutrient-rich topsoil and humus. This can cause a reduction in crop productivity on agricultural lands and biodiversity in natural areas.

The plains of the United States are very windy due to their flat topography. There is not much to block the wind, allowing wind to blow freely. These areas are also where most of the agriculture in the United States is performed, and planting and plowing usually happens during the dry season. This means that if farmers are not careful, valuable nutrient-rich soil can be blown away if it is very windy.

Effects of Glacial Erosion

Glacial erosion gradually shapes the land over thousands of years. Picture a glacier as a giant, slow-moving bulldozer made of ice, pushing rocks, soil, and debris along as it creeps down a mountainside. The effects of glacial erosion can be seen in the dramatic valleys, sharp peaks, and deep lakes that glaciers leave in their wake. Next time you see a majestic mountain or a serene lake, remember that it might just be the handiwork of a patient glacier!

Essential Knowledge

Soils can be eroded by winds or water. Protecting soils can protect water quality as soils effectively filter and clean water that moves through them.

AP Exam Tip

Soil includes both biotic and abiotic components.

You should know different soil horizons, especially which is on the top, and which is the bottom.

You should know the types of erosion.

You should know the effects of both wind and water erosion.

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