1. Soil
& Soil Horizons

2. Learning Objectives 1. We will identify what soil consists of.
2. We will describe how climate and landforms affect a soil’s characteristics.
3. We will describe how the activities of organisms affect a soil’s characteristics.
4. We will explain how the properties of soil differ.
Why is it important to learn about soil?

3. Have you ever . . . Dug a hole in the ground?
What did you find? Did you notice any differences?

4. What is soil? A mixture of 5 materials: Weathered Rock Particles Water1
What is soil?
A mixture of 5 materials:
Weathered Rock Particles
Main ingredient
Water
Air
Minerals (example Iron)
Organic Matter (called humus)
Comes from the remains and waste products of plants, animals, and other living organisms

5. All soils are not the same.1
All soils are not the same.
A soil’s composition determines what you can grow in it, what you can build on it, what happens when rainwater falls on it.
The kind of soil that forms in an area depends on:
The kind of rock in the area
The area’s climate
The landforms in the area
The plants in the area
The animals and other organisms in the area
Time

6. Compare and Contrast
Desert Soil
Grasslands

7. Soil Horizons Soil develops in a series of horizontal layers.1
Soil develops in a series of horizontal layers.
Soil Horizon – a layer of soil with properties that differ from those of the layer above or below it.

8. Soil Horizon Layers O Horizon – A Horizon –1
Soil Horizon Layers
O Horizon –
accumulation of twigs, leaves, and decomposing plants
A Horizon –
Top soil layer
Contains humus mixed with mineral particles
Darkest color

9. Soil Horizon Layers B Horizon – C Horizon – R Horizon –1
Soil Horizon Layers
B Horizon –
Has very little organic material
Contains clay and minerals washed down from A Horizon
Brownish or reddish in color
C Horizon –
Deepest layer of soil
Contains the largest and least-weathered rock particles
Light yellowish brown in color
R Horizon –
Layer of Bedrock

10. Climate and landforms affect soil2
Climate and landforms affect soil
Different types of soils form in different climates.
Tropical: heavy rains wash away minerals and a thin layer of humus; not suitable for crops
Desert: shallow and contain little organic matter; low rainfall means soil formation occurs slowly
Temperate: moderate rainfall allows for dark-colored soil rich in organic matter that is good for crops.
Arctic: cold, dry regions with poorly developed horizons and a large amount of rock fragments

11. Climate and landforms affect soil2
Climate and landforms affect soil
Landforms affect how soil is developed.
Example - Mountains vs. Valleys
Cold climate on a mountain means a slower forming soil that often washes down the mountainside into the valley, resulting in a thicker A Horizon in the valley.

12. Living organisms affect soil3
Living organisms affect soil
Plants – provide most of the organic matter that forms humus when they lose their leaves or die and decompose
Microorganisms – decompose dead plants and animals and produce nutrients that other plants need to grow
Animals – loosen and mix the soil as they tunnel through it, adding air and improving its ability to absorb water

14. 4
Soil Properties
Texture – determined by the size of the weathered rock particles it contains
Fertility – The ability of a soil to provide plant nutrients.
Pore Space – amount of space between the soil particles
Chemistry – pH level of the water in the soil (how much acid is in the water)
Water Retention- The soils ability to hold water/moisture.

15. Soil Texture and pH
Texture of a soil is determined by the amounts of sand, silt, and clay it contains.
Sand – largest, water moves through quickly
Silt – medium, holds more water than sand
Clay – smallest, absorbs the most water
pH level in soil affects how well the nutrients in the soil are dissolved by water.
Why would a farmer care about the PH level of his/her soil?