1. Soil
Soil is a mixture of organic matter and broken down rocks. Often containing sand, clay, and water, it acts as an ecosystem for thousands of organisms.

2. SOIL: Horizons
Figure 3-23 2

3. Soil has different layers called “Horizons”
• A horizon: Also known as top soil.
→ Decomposed organic matter.
→ Highest level of nutrients.
→ Thousands of bacteria live here.
→ Shallow plant roots and fungi.
B horizon: Also known as subsoil.
→ Good mix of organic matter eroded rocks. Less nutrients / life compared to the top soil.

4. Soil has different layers called “Horizons”
• C horizon: a.k.a parent material.
→ High amount of rocks + minerals. Not that much organic matter.
Bedrock
→ Large Rock slabs that haven't eroded yet. The bottom origin of the soil. Tree roots usually stop above this point.

5. Examples of Soil Profiles
Figure 3-24 5

6. Formation of Soil

7. Formation of Soil
• First, bedrock must erode to free up nutrients that algea, lichen, and bryophytes such as moss need to grow.
WEATHERING / EROSION
→ The break down and translocation of the Earth's crust by both natural processes such as weather and the activity of living organisms.

8. Formation of Soil
• Second, decomposing organisms deposit a layer of top soil (horizon A)
• Maturity of the Soil:
→ The subsoil is the last layer to form.
→ Organic matter from the top mixes with parent material from the bottom.
→ B horizon grows over time.

9. Sources of Parent Material
Weathering or erosive actions:
heating/cooling
freezing/thawing
glaciers
water
wind
chemistry
plants & animals

10. Nutrient-rich Top Soil
The layer of decomposing organic matter in the top soil is called “Humus.” More humus = more nutrients.

11. Soil Separates Most soils have a combination of soil particles sizes
Sand
Silt
Clay

12. Particle Sizes Clay: less than 0.002 mm Silt: 0.002-0.05 mm
Sand: mm
mm fine
mm medium
mm coarse
1- 2 mm very coarse
Gravels: 2-75 mm
Cobbles: mm
Stones: mm
Boulders: >600 mm

13. Soil Nutrition

14. Nutrients in Soil
Studies show that there are between 12 – 14 nutrients that plants need which they can only obtain from the soil. 

15. Primary Nutrients
Plants need certain nutrients in very large quantities in order to grow + reproduce.
The primary nutrients are nitrogen (N), phosphorus (P), and potassium (K).

16. Soil pH is a factor
Soil pH is one of the most important soil properties that affects the availability of nutrients. 
Nutrients tend to be less available in soils with very low or very high pH.

17. Risks to Soil: Exhaustion
Agriculture (large-scale farming) in an area can EXHAUST the soil.
Growing crops in the same area that need the same nutrients year after year drains that area of those nutrients.

18. Risks to Soil: Erosion
The removal of trees from an area increases SOIL EROSION.
Tree roots help keep the soil in place. Without them, topsoil can be blown/washed away.

19. Risks to Soil: Pollution
Human activity / waste deposits chemicals into the soil. These chemicals can be toxic to soil organisms.
Heavy metals such as Pb and Hg.
Strong acids + bases.