1. Soil Formation
Lesson 3

2. Why do we need soil?

3. How does soil form?
First, weathering breaks solid bedrock into smaller pieces.
Then, those pieces continue to undergo weathering and break down even more.
Next, worms and other organisms help break down organic matter and add the nutrients from them to the new soil. These organisms also create passages through the new soil for air and water.

4. Where does new soil come from?
Most of the soil is formed from the weathered products of a rock, called the parent material.
A soil whose parent material is the local bedrock is called residual soil.
Transported soil is soil that develops from parent material that has been moved far from its original location, usually by agents of erosion.

5. Is all soil the same?
New soils that do not yet have layers are called undeveloped soils.
Mature soils have distinct layers, called soil horizons.
There are typically four major soil horizons in mature soils: O, A, B, and C.

6. What is in each layer of soil?
The O-horizon is the top layer of organic material and is composed of humus and leaf litter.
The A-horizon is a layer of weathered rock combined with a rich concentration of dark brown organic material.
The B-horizon, also known as the zone of accumulation, is a red or brown layer enriched over time by clay and minerals deposited by water flowing from above, or percolating upward from below.
The C-horizon contains little or no organic matter and is often made of broken-down bedrock.
Extension:
What is the soil horizon, “Caliche”?
Is this good for plant growth, or bad? Explain.
What regions have caliche soil horizons?

7. Why do some locations have better soil than others?
Five factors influence soil formation: climate, topography, parent material, biological organisms, and time.
These factors combine to produce 12 different types of soil, called soil orders.

8. Which factor has the greatest impact on forming new soil?
Climate, which includes temperature, wind, and the amount rainfall, is the most significant factor controlling the development of soils because it also affects the weathering of rocks.
Apply what you learned in our last lesson:
What kinds of temperature would form new soil quickly?
What kinds of wind would form new soil quickly?
What amount of rainfall would form new soil quickly?

9. Why are there fewer plants on steep slopes, but plenty of plants at the bottom of these slopes?

10. Why are there fewer plants on steep slopes, but plenty of plants at the bottom of these slopes?
Topography, which includes the slope and orientation of the land, affects the type of soil that forms.
Soils cannot develop if the weathered material is moved away by erosion, so there is not much soil on steep slopes.

11. Why is soil not the same everywhere?
Soil can only contain the same minerals and chemicals as the rocks that it came from.
If the soil is residual, it will have the same chemical composition as the local bedrock.
If the soil forms from transported rocks and minerals, the minerals in the soil are likely to be different from those in the local bedrock.

12. Can we have good soil without living organisms?
Organisms including fungi and bacteria, as well as plants and animals, interact with soil.
Different types of biological organisms in a soil can result in different soil orders.
Biological organisms are responsible for adding organic compounds and chemicals to the soil. Without these organisms, the soil would not contain the necessary components to sustain plant life.

13. How does soil change over time?
After years of weathering, most of the original minerals in mature soil are changed or washed away.
New soils take a long time to develop into mature soils that can support plant life

14. How do plants and sunlight affect where soil is found?

15. What does the size of the soil particles have to do with growing plants?
The particle sizes found determine a soil’s texture, which affects its ability to retain moisture and therefore its ability to support plant growth.
A soil textural triangle (next slide) is used to classify a soil’s texture.

16. Activity: Go outside and collect soil samples:
Loam is the best garden soil, but different plants have different requirements.

17. What makes the best soil for growing plants?
Although different plants have different requirements, there are some general rules for what makes good soil for plants:
Topography (no steep slopes, flat ground is usually best)
Availability nutrients (plants need nitrogen, phosphorus, and potassium to grow)
The number of microorganisms present (high enough to provide lots of organic material and make tunnels for water and developing plant roots)
The amount of precipitation available (moderate amounts are best: enough to water the plant without flooding it)
Acidity (most plants like soil that is close to neutral)
Extension Questions:
What are the effects of nitrogen on plants? Phosphorus? Potassium?
If your soil is low in nutrients, is it better to use chemical or organic fertilizers? Explain.
Soil low in organic material can be replenished by adding compost. What is composting, and how could you start composting now?

18. What makes soil different colors in different places?
The minerals, organic matter, and moisture in each soil horizon determine its color.
Scientists use a book of color chips, known as the Munsell System of Color Notation, to describe soil color
It breaks color down into: hue (color), value (lightness or darkness), and chroma (intensity).
This information has been used to answer a wide variety of real world questions, from archaeology to criminal investigations, since soil colors are strongly connected to specific geographical locations.
Extension Questions:
1). What color soils are best for rich plant growth? Why?