1. The Scoop on Soil
We seldom think about the importance of soil. When we do think of it, we usually say it’s just dirt. However, soil is not the same as dirt. What’s the difference between soil and dirt? Dirt is what you find under your fingernails. Soil is what you find under your feet. Think of soil as a thin living skin that covers the land. It goes down into the ground just a short way. Even the most fertile topsoil is only a foot or so deep. Soil is more than rock particles. It includes all the living things and the materials they make or change.
Soil is an important fundamental resource of life. It covers large parts of the dry land surface of the Earth. Without soil, Earth’s surface would be just barren rock and sand, which could not support life.
Soil supports many communities of living things which are interdependent, exchanging energy and chemical resources with each other. In one shovelful of fertile dirt, there are more living creatures than there are human beings on our entire planet.

2. Of What is Soil Made? sand rocks dried plants dead organisms water air
Soils are a mixture of different things; rocks, minerals, and dead, decaying plants and animals. Soil can be very different from one location to another, but generally consists of organic and inorganic materials, water and air. The inorganic materials are the rocks that have been broken down into smaller pieces. The size of the pieces varies. It may appear as pebbles, gravel, or as small as particles of sand or clay. The organic material is decaying living matter. This could be plants or animals that have died and decay until they become part of the soil. The amount of water in the soil is closely linked with the climate and other characteristics of the region. The amount of water in the soil is one thing that can affect the amount of air. Very wet soil like you would find in a wetland probably has very little air. The composition of the soil affects the plants and therefore the animals that can live there.

3. Soil Stacks Up
Organic Layer: Dead plants and animals settle on the surface of the ground to break down.
Topsoil: Plants grow and animals live on top of the soil. This is sometimes called the organic layer. A thick cover of plants can keep the soil cool and keep it from drying out. Decomposers recycle dead plants and animals into humus.
Subsoil: This is a mix of mineral particles and some humus near the top. Subsoil is very low inorganic matter compared to the topsoil. This is the layer where most of the soil's nutrients are found. Deep plant roots come here looking for water. Clays and minerals released up above often stick here as water drains down.
Weathered parent material: This horizon can be very deep. There's no organic matter here at all. We're out of reach of all living and dead organisms down here. It's all rock particles, full of minerals.
The entire soil profile used to look like this all the way to the surface. Physical weathering broke the parent material up into small pieces. Don't be fooled! This layer may contain rock particles that are different from the bedrock below. A river or a glacier might have brought it from somewhere else.
Bedrock: We finally found solid rock! The bedrock formed before the soil above it. It will wait here until erosion or an earthquake exposes it to the surface. Then some of it will be weathered to become the next batch of parent material. The soil-making process will start all over again.

4. What are Some Processes that Form Soil?
Decay of plant matter
Settling of volcanic ash
Transport by rain, streams, and rivers
Deposition of sediments in rivers and lakes
Weathering
Erosion
We will look at each of these processes in the next few slides to see how they contribute to the formation of soil.
Weathering and erosion slowly chisel, polish, and buff Earth’s rock into works of art and then wash the remains into the sea. The processes work together, but they are independent of each other.
Weathering is the breaking down of rock into pebbles and sand caused by physical processes, such as heating, cooling, and pressure and chemical processes such as acid rain. Erosion is the transporting of sediments. It is the movement of Earth materials by processes such as wind, water, ice, and gravity.

5. Decay of Plants
In a forest, leafy litter and woody materials fall to the ground. When it decays so much that it isn’t recognizable, it becomes organic matter.. The organic matter continues to be broken down until it is humus. Humus comprises the decayed organic matter in the soil.

6. Settling of Volcanic Ash
Volcanic ash, from an eruption can be carried thousands of miles away by the wind. As it falls to Earth, it becomes part of the soil. Most volcanic ash forms into very fertile soil that is used for growing crops, although too much falling ash can kill plants.

7. Transport by Rain
When you have a slope and it rains, there will be drainage. The runoff carries away small rocks and minerals. This runoff winds up in valleys or in the ocean. It slowly builds up and the small pieces make soil.

8. Deposition of Sediment
Deposition is carrying particles to a resting place. Sediment is particles that are transported by fluid flow and eventually deposited as a layer of solid particles on the bottom of a body of water. This picture shows deposits of sediment in layers along a river bank. At one time, the water was higher, and particles settled at the bottom. As the stream level lowered, it left these deposits of sediment behind.

9. Weathering

10. There are Two Main Types of Weathering
Mechanical Chemical
Again, weathering is the process of breaking down rocks.
 Mechanical weathering is the process of breaking big rocks into little ones. This process usually happens near the surface of the planet. Temperature also affects the land. The cool nights and hot days always cause things to expand and contract. That movement can cause rocks to crack and break apart. Roots and plants also push into the rocks and break them apart. They act like wedges and push the rocks apart. Little animals also help by burrowing and digging through the ground. Scientists have observed a process called freeze-thaw. That process occurs when the water inside of rocks freezes and expands. That expansion cracks the rocks from the inside and eventually breaks them apart. The freeze-thaw cycle happens over and over again and the break finally happens. Another word for it is frost wedging. There are already some small cracks in boulders and water can get in the cracks. When that water expands, the rock is crunched between two expanding pieces of ice instead of being forced apart into separate pieces.
Another type of mechanical weathering is called salt wedging. When it rains and water flows everywhere, it usually has ions and salts dissolved inside. Have you seen salt water dry up? Salt crystals are left over. Those crystals happen in nature, too. The water flows in a rock and evaporates. Crystals slowly begin to grow. Those crystals act like a wedge and crack the rocks.
Chemical weathering includes the effect of weathering on molecules and atoms. As with all chemistry, the greater the surface area of an object, the more chemical reactions can take place. For these chemical reactions to happen in nature, moisture, and heat must be present. Reactions such as oxidation, hydrolysis, and acidification can happen when all of the elements are together. Oxidation makes rocks softer. It is similar to an iron bar rusting. Since there is a lot of iron in many rocks, oxidation often happens. Hydrolysis usually causes rocks to expand and then mechanical weathering can begin. These chemical reactions are happening all of the time. When you see rocks next to each other that are different colors (often shades of red) then you know chemical reactions have taken place.

11. In the next 7 slides, try to determine what caused the weathering.
Mechanical. Wind has worn away the rock by battering it with sand.

12. Mechanical. Organisms bored into this rock.

13. Chemical. Rainwater mixes with chemicals as it falls from the sky, forming an acid that dissolves rock. Acid rain dissolves limestone for form caves like this.

14. Mechanical and Chemical
Mechanical and Chemical. Lichens and mosses squeeze into cracks and crevices, where they take root. When they grow, so do the cracks which eventually split into bits and pieces. That’s mechanical weathering. Plants and animals also produce acids that mix with rainwater, making a combination that eats away at rocks. That’s chemical weathering.

15. Mechanical. Freeze-thaw occurs when water continually seeps into cracks, freezes and expands, eventually breaking the rock apart.
On a frigid day, water pools in the cracks and crevices of rocks. The temperature drops at night and the water expands as it turns to ice, causing the rock to split. Then during the heat of the next day, the ice melts and trickles the cracked fragments away.

16. Mechanical. Wind has sculpted the rocks by blowing sand
Mechanical. Wind has sculpted the rocks by blowing sand. Bits of sand, carried by the wind, can blast the sides of nearby rocks, buffing and polishing them.

17. Mechanical. Wind, sand, and water.

18. Erosion http://video.yahoo.com/watch/1794382/5933084
Remember that weathering breaks down and sculpts the rocks. Erosion is the process that transports the fragments of broken down rocks away. The movement can take place by gravity or by a moving transport agent, like wind, water, or ice.

19. Erosion from Gravity
Again, erosion is the movement of earth materials by processes such as wind, water, ice, and gravity. Erosion is transporting the weathered particles away.
This photo shows down slope movement of loose rock and soil from gravity. Landslides can be sudden and are often aided by water.

20. Erosion by Wind
A strong breeze can easily pick up sand and dirt particles and carry them hundreds of kilometers away.

21. Erosion by Water
Around the world, moving water picks up and transports millions of tons of sediment every day, along rivers, coasts, and even in the deep oceans.

22. Erosion by Ice
A glacier is a river of ice with great erosive power. Broken rock gets stuck in the ice and carried away.

23. What’s good about erosion?
New soil is formed. Erosion results in deposits of dark, fertile soil, perfect for growing crops.
Erosion reveals marvels of nature such as mountains, sandstone arches, and polished cliffs.
Without erosion, rock debris would simply pile up.
Soil erosion may slow down global warming by carrying carbon into wetlands where it is stored for long periods of time. (theory)
There are probably more that students can think of.

24. What’s bad about erosion?
Land can be stripped of soils needed for food to grow.
Landslides can destroy whatever is in their paths.
Rivers can be filled with silt, disrupting their ecosystems.
Erosion causes cracks, gaps, and ditches in the landscape.
When soil erodes, it cannot support many types of plant life.
There are probably many more students can think of.

25. What can be done to prevent erosion?
Plant trees and bushes along riverbanks to hold soil in place.
Plant trees that will block the wind.
Use careful, smart farming practices that won’t disturb topsoil.
Use common-sense approaches to construction, such as don’t build on steep slopes.
When hiking, stay on trails.
Students may be able to think of more.

26. Review 1. What is weathering?
Answer: Weathering is breaking down rock into pebbles and sand by physical or chemical processes.

27. 2. Give at least three examples of weathering processes.
Answer: Wind, water, ice, heating, cooling, acid rain.

28. 3. What is erosion?
Answer: Erosion is the movement of Earth materials by processes such as wind, water, ice, and gravity.

29. 4. Explain how the formation of soil is related to weathering of rock.
Answer: Rocks are broken into small pieces by the process of weathering. Then the small particles become part of the soil.

30. 5. How does deposition of sediments relate to soil formation?
Answer: As sediments fall to the bottom of rivers and lakes, they build up deposits to form soil.

31. 6. Name at least two forces that cause erosion.
Answer: Wind, water, gravity.

32. 7. What is soil?
Soil is the thin living skin that covers the land. It goes down into the ground just a short way. Soil is more than rock particles. It includes all the living things and the materials they make or change. Soil is a precious resource that sustains life on earth.

33. 8. Name at least three layers of soil.
Organic layer, topsoil, subsoil, rock

34. 9. What can you personally do to help prevent erosion?
Plant trees and bushes along riverbanks to hold soil in place.
When hiking, stay on trails.

35. What process causes landslide erosion?
gravity

36. Credits The Rock Cycle http://www.geolsoc.org.uk/rockcycle
Soil Forming Factors
Dirt: The Scoop on Soil (Amazing Science) [Paperback]
Rosinsky, Natalie M (Author)
The Open Door Website
Soil Profile
Soil is not dirt
The Dirt on Soil
What is soil?
Geography4Kids.com
Tree Hugger
Deadly Landslides in Guatamala
Wikipedia
Weathering and Soils
Soil Forming Factors
How Stuff Works
North Coast Journal
Just the facts