It’s not just DIRT

Soil

Soil is a natural resource. A natural resource is a material on Earth that is necessary or useful for people. It is not man made.

Because it is made from the weathering of rocks, it is considered a “renewable” resource.

Without soil, plants could not grow, which means people and animals would have no food!

Why is soil important?

1. It is the main product of mechanical and chemical weathering

2. Bacteria breakdown of plants and animals forming HUMUS

3. Worms, ants, moles, burrow through soil allowing water to weather rock and supplying nutrients.

How is soil formed?

25%

45%

25%

5%

What is Soil?

Air Mineral MatterWater Organic Matter

1. Parent material/Bedrock2. Time: the more time soil has to develop, the

thicker the soil, the less is resembles the parent material.

3. Climate: most influential factor4. Plants and Animals: fertility depends on

amount of organic matter5. Slope:

1. Steep slopes: poorly developed

What controls soil formation?

Topography

What makes up a Soil profile?

A soil profile is a vertical sequence of soil layers

A soil profile is made up of horizons. The number of horizons depends on how “mature” the soil is. “Mature” soils take 1000’s of years to form and contain about 6 horizons. “Immature” soils sometimes only contain 2 horizons.

How are soil profiles formed?

What makes up a soil profile?

The “O” Horizon: Topsoil, mostly plant litter at the top and humus at the bottom. Rich in organic material. Brown in color.

The “A” Horizon: Topsoil, dark in color, some humus, and biological activity

The “E” Horizon: Light colored mineral materialLeaching: Water percolates down and

carries minerals down to lower horizons.

What makes up each of the soil horizons?

The “B” horizon: Subsoil, contains clay material. Red-brown color.

The “C” horizon: partially altered parent materialRegolith: rock and mineral fragments, NO

organic materialResidual soil: soil formed over the original

bedrock. Resembles the bedrock/parent rock.

The “R” horizon: Solid bedrock/parent rock, unweathered

What makes up each of the soil horizons continued

Different soils are found in different places. Some soil is good for growing plants, while other soil is not.

Soils differ in color and texture. A soil’s color has to do with what’s in it. For example, red soil may contain a lot of iron. Dark brown or black soil contains a lot of humus.

How are soils different?

Soil TypesClimate/Soil

Type

Vegetation

Typical Area (s)

Common colors

Chemistry and

Composition

Annual Rainfall

Fertility

Soil texture describes how big the particles, or pieces, of soil are.

Most soils are either sand, clay, silt, or loam.

What are the different soil textures?

Comparing Soil Layers

Sand: the largest particle in the soil. Water flows through it very easily. Because it doesn’t retain water very well, plants have trouble growing in this type of soil.

Clay is the smallest of particles. Clay also can hold a lot of nutrients, but doesn't let air and water through it well. Can result in TOO MUCH WATER for the plant.

What are the different soil textures?

Silt is a soil particle whose size is between sand and clay. Smaller than sand, larger than clay.

Loam: This soil is a mixture of sand, silt, clay and humus. It is considered to be the perfect soil. Holds water well and has many nutrients.

What are the different soil textures?

Porosity is a measure of how much of a rock is open space. This space can be between grains or within cracks or cavities of the rock.Permeability is a measure of the ease with which a fluid (water in this case) can move through a porous rock.

What is the difference between porosity and permeability?

The soil texture triangleSoil texture depends on its composition and the relative portions of clay, sand, and silt. 

Classify a soil sample that is 30% clay, 15% silt, and 55% sand.

How to read the soil triangle

 60% clay, 20% silt, 20% sand

Now you try!

Clay: 30% Sand: 30% Silt: ?

Now you try

Soil is eroded when it is moved from place to place

It is more common on steep slopes or where there is little vegetation

Balance is maintained when new soil forms at the same rate it is eroded

How is soil eroded?

Wind erosion

Raindrop erosion

Vegetation protects soil

1. Agricultural practices such as plowing and slash and burn removes plant cover

2. Clear cutting for lumber, farming, and grazing3. Overgrazing by sheep and cattle4. Urban construction5. Open strip mining

How do humans cause erosion ?

No-till farming is done without plowing leaving topsoil in place

Contour and Terracing uses the natural shape and slope of the land to grow crops

Rotation system

What can we do to slow the rate of soil erosion?