Soil is formed through the weathering of rock and mineral particles and the accumulation of organic matter over long periods of time. The document discusses the key components and layers of soil, including the O horizon of organic material, the A horizon containing humus and nutrients, the B horizon containing clay and minerals, and the C horizon containing broken down bedrock. Several factors influence soil formation, such as climate, topography, parent material, biological activity, and time. The texture and permeability of soil is determined by the relative proportions of sand, silt, clay, and humus particles.

1. Soil

2. Soil Formation
• Soil is a mixture of rock and mineral particles and organic
matter.
• Soil is made up of mineral matter that has been broken down by
chemical, physical, and biological actions.
• There are also living things in soil such as bacteria, fungi, mold,
worms, and insects.
• The more living things the more productive the soil becomes.

3. Facts on Soil
- Soil is the #1 supporter for plants
- Soil anchors the plant roots so they don’t fall over
- Soil is a superstore of nutrients
- Moisture for plants is stored in soil.

4. Soil Layers
• During the process of soil formation, layers develop in the soil.
• Most of the volume of soil is formed from the weathered products
of a source 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. Transported Soil –
Removed from its
original location
Residual Soil – parent material is local bedrock

6. Soil Profiles
• A soil profile is a vertical sequence of soil layers.
• A distinct layer within a soil profile is called a soil horizon.
• There are typically four major soil horizons in mature soils: O, A, B,
and C.

7. • 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.

8. Factors of Soil Formation
• Five factors influence soil formation:
• Climate
• Topography
• Parent material
• Biological activity
• Time.

9. Climate
Climate is the most significant factor controlling the development of soils
because it also affects the weathering of rocks. Temperature, wind, and
the amount of rainfall determine the type of soil that can develop.
Topography
Topography, which includes the slope and orientation of the land, affects
the type of soil that forms.
Slopes that face south receive more sunlight than other slopes, allowing
more vegetation to grow.

10. Parent material
If the soil is residual, it will have the same chemical composition as the
local bedrock. If the soil is transported, the minerals in the soil are likely to
be different from those in the local bedrock.
Biological activity
Organisms including fungi and bacteria, as well as plants and animals,
interact with soil. Different types of organisms in a soil can result in
different soil orders.
Time
The effects of time alone can determine the characteristics of a soil. After
tens of thousands of years of weathering, most of the original minerals in a
soil are changed or washed away.

11. Soil Texture and Color
For soil, the two main physical properties are color and texture
Color
The minerals, organic matter, and moisture in each soil horizon
determine its color.
• The darker the soil, the more nutrients it contains (dark soil
contains humus)
• Gray soil indicates poor drainage and red soil indicates a lack of
nutrients

12. Soil Texture
The relative proportions of particle sizes determine a soil’s texture, which
affects a soil’s capacity to retain moisture and therefore its ability to support
plant growth.
A soil textural triangle is used to determine a soil’s
texture.
Knowing the soil’s texture you can find:
1. How easy water flows through it
2.Water holding capacity
3. How well the plant will grow in it

13. Soil Permeability and Porosity
The particle size that make up each type of soil determines the size of
pores between particles. This is known as the soil’s porosity. These
the soil hold water and air.
The larger the particles making up the soil,
larger the pores are between them.
Particle size (Largest to Smallest)
Sand  Silt  Clay
The soil’s permeability is how easy fluids can
flow through its particles.

14. • Sand
• Feels gritty
• Does not stick together in a mass unless wet
• Has fewer nutrients for plants than silt and clay
• Pores between sand particles allow free drainage
of water and entry of air
• Holds little water, prone to drought
Silt
• Feels floury powdery – smooth like putty when wet
• Wet silt does not stick together and cannot mold into different shapes
• Smaller particles than sand, retains more water for plants
• Drains slower than sand
• Easily washed away by flowing water

15. • Clay
• Powdery when dry, sticky/slippery when wet
• Wet clay can be molded into any shape
• Swells when added to water, shrinks and becomes hard when water
evaporates
• Pore spaces are very small
• Water/air move very slowly through clay
• Tremendous ability to hold water

16. • Humus
• Formed when dead plants and animals decay, which provide essential
nutrients for plant growth.
• Dark brown or black (color)
• Feels crumbly and loose when dry and spongy when wet
• When dead plants and animals decay, humus is
accumulated in the second layer of soil (A)
• Holds more water than sand and silt but
not has much as clay. Water will drain quickly.