1. Soil is a mixture of Humus, Sand, Silt and Clay
What is soil?
Soil is a mixture of Humus, Sand, Silt and Clay

2. Erosion and decomposition
Soil is what happens when:
Erosion breaks down rocks into smaller pieces (From the bedrock up).
Decomposition occurs. Living things die and decompose and mix in (From the top down).
Layers of soils are called horizons.

3. Soil Profile Litter (Humus only) Horizon A
(Humus and some crumbled rocks)
Horizon B
(crumbled rocks and some humus)
Horizon C Subsoil
(Crumbled rocks only)
Bed Rock

4. Soil types and particle sizes
The soil types are determined by:
the particle sizes
their ability to retail water
clump together

5. Sand Sand pieces > 50µm in diameter
Large particles and very porous when you hold them in your hand they slip through your fingers.
It lets water flow through quickly and
Has low water retention.

6. Silt Silt pieces are in between 50µm > x > 2µm
Mid sized particles pores are smaller than sand. It clumps when you squeeze it in your hand. The particles are small enough to be carried by the wind or water. It contains lots of minerals.
It lets some water through and
It retains some water.

7. Clay Clay pieces < 2µm
These are the smallest particles. They stay squeezed together in your hand like play dough. There is not much space for air or water to circulate.
It retains water.

8. The Champlain Sea

9. Our soil
The St Lawrence River valley was once at the bottom of the Champlain Sea.
It’s some of the richest farmland in the world.
It’s full of clay and silt as well as all the detritus
( Excrement and dead bodies of stuff that falls to the bottom).

10. Porosity Porosity: Refers to the amount of space in the soil between.
If there is a lot of space between the particles, then the water and air can circulate freely.
If the pores are small, then the soil retains water.

11. Leeching
Leeching: When particles of rock are soaked in water the minerals in them dissolve into the water.
Examples of minerals: Mg, Ca, Fe.
Plants need Magnesium and Calcium and Iron to grow.

12. Decomposition
Decomposition: The chemical change that takes living things (now dead) and turns them back into their Non-living nutrients.
This is accomplished by worms, insects, fungus and bacteria.
Examples of nutrients: Nitrogen (N), Phosphorous (P), Potassium (K).

13. What plants need
Plants need soil that contains nutrients. They need N for leaves, P for fruits and flowers, K for roots.
They need soil that contains minerals for water movement and growth.
They need soil that retains water as that is one of the inputs of photosynthesis and it dissolves the minerals and nutrients so the plant can absorb them.
They need soil that has some air in it for gas exchange.

14. Homework
What do you think is the best combination of soils to growth plants?
Design the perfect potting soil.