1. Soil Formation and Composition
Chapter Seven
Soil Formation and Composition 1

2. What is soil?
Soil - combination of mineral and organic matter, water, and air
It is that portion of the weathered rock and mineral that supports the growth of plants
Soil begins as rock is broken down and mixes with other materials. 2

3. Soil is formed wherever bedrock is exposed to weathering.
How is soil formed?
Soil is formed wherever bedrock is exposed to weathering.
Bedrock – solid layer of rock beneath the soil. 3

4. What makes up soil?
Rock, minerals, decayed organic material, air & water.
The type of rock depends on:
Type of bedrock 2. Type of weathering
Humus = decayed organic material
- makes space for air & water in soil.
- nutrient rich promote plant growth 4

5. Components in soil that support plant growth
Mineral Matter
Silt 18%
Sand 18%
Clay 9% 5

6. What determines how soil feels?
Soil is made up of particles which vary in size.
Gravel
Sand
Silt
Clay
The amount of particles in the soil determines how well certain plants will grow. 6

7. Too Much Gravel & Sand?? = soil won’t hold much water.
Too Much Clay?? = soil will be heavy and dense, may hold too much water and have not enough air.
Loam = soil that is made of equal parts of clay, sand & silt.
Loam is best for growing. 7

8. Soil Horizon or Profile
Soil develops over time and forms layers or horizons.
A Horizon = Topsoil
B Horizon = Subsoil
C Horizon = Partly weathered rock.
Topsoil = crumbly dark soil = mixture of clay, humus and minerals
Subsoil = Minerals washed down from above = mostly clay 8

9. An idealized soil profile with horizons9

10. 10

11. 11

12. What determines how fast soil forms?
1. Climate 2. Type of rock.
Soil = weathered rock and particles
Weathering happens most quickly in hot, wet climates.
These climates will make soil the quickest.
What type of climate will make soil the slowest? 12

13. Review 1. What is soil? 2. How is soil formed?
3. Why isn’t a soil lots of clay good for plant growth?
4. What is humus? What is loam?
5. What are the three basic horizons in the soil profile?
6. What determines the rate of soil formation? 13

14. What is happening in the soil?
Organisms that live in the soil mix the soil and create spaces for water and air. (moles, mice, chipmunks).
Some organisms make humus which makes soil rich. (plants die).
Some animals are decomposers – break down dead material (bacteria, fungi, worms) 14

15. 15

16. 16

17. Other animal activities???
Don’t forget that some soil is very rich in…
…… Fertilizer …. Animal waste …. which is rich in nitrogen ….
…. which helps plants grow …. Aren’t you glad
… you’re not a plant? 17

18. What kind of soil is in the USA?
Soils vary greatly across the US and are dependent on climate and type of rock. 18

19. Equator to Poles Factors
Annual precipitation
Temperature
Equatorial
and tropical
rain forests
Low-latitude
deserts and
semi-deserts
Temperate
regions
and mixed
boreal forests
Arctic
and
tundra
regions
Grasslands
(steppes)
Savannahs
30 degrees Latitude
40ºC
Precipitation
1800 mm
Temperature
30ºC
Evaporation
20ºC
600 mm
10ºC
Bedrock at
or very near
surface
Soil
Soil
Deeply weathered
bedrock
(~ meters deep)
Bedrock
of weathering
Increasing depth 19