Soil Formation

Soil• Soil is the loose covering of broken rock

particles and decaying organic matter overlying the bedrock of Earth’s surface.

• Where does it come from? – Weathering

• Mechanical and Chemical – Accumulation of detritus

It takes thousands of years for soil to form.

Above and Below Factors Influencing Soil

Parent Material & Organisms• Soil composition is dependent on the parent

rock. – The parent rock is the underlying rock material from

which the inorganic components come.

• Organisms begin to live in the pieces of rock. – Bacteria, fungus, worms,

insects.• Add nutrients to form soil. • Mix it up!

Climate• The climate influences the type of soil because

it determines the type of weathering that occurs. – Mechanical weathering dominates in colder

climates. • Soils are not well developed and are very similar to

parent rock. There is very little to no decayed organic material.

– Chemical weathering dominates in tropical climates. • Soils are well developed. They have more leaching

of nutrients and decomposition of detritus.

Climate Influences Soil Type Time & Topography

• Soils formed in steep topography is constantly removed by gravity. – Soil accumulates in the

lowlands.

• The more time a soil has to develop, the more it takes on the characteristics of its given biome. – Ex: Grasslands have very fertile soil from the

vegetation inputs.

The Soil Profile• Soil forms in layers known as horizons. Each

horizon has its own characteristics.Let’s model them!

What are the Layers?

What are the Layers?O: Contains organic detritus such

as leaves, twigs, needles, etc.

A: Mixture of minerals and humus

B: Nutrients are here, but little organic material. Usually clay.

C: Partially weathered rock

R: Parent Rock

E: Zone of leaching

What’s the Order?• Which of the following puts the horizons in

order of formation?1. C, B, A

2. A, B, C

3. C, A, B

4. B, C, A

Soil Properties

Importance of Soil

Which Property…• …describes the amount of space between

the grains of a sediment.A. Texture

B. Porosity

C. Permeability

D. Water Retention

Which Property…• …describes the relative amounts of sand,

silt, and clay.A. Texture

B. Porosity

C. Permeability

D. Water Retention

Which Property…• …describes the ability of a soil to let water

pass through it.A. Texture

B. Porosity

C. Permeability

D. Water Retention

Which Property…• …describes the ability of a soil to hold

water.A. Texture

B. Porosity

C. Permeability

D. Water Retention

Chemical Properties• Clay particles contribute to the chemical

properties of a soil because they have a negative charge.Allows them to attract positive cations!

• These cations are adsorbed to the grains. • Can be subsequently release as nutrients

for plants.Measured as cation exchange capacity (CEC)

Chemical Properties• CEC is the ability of a particular soil to

adsorb and release cations. • Also known as nutrient holding capacity.

The more clay, the higher the CEC!

• Too much clay is not a good thing! • More than 20% means water retention is

too high.

Chemical Properties• Base saturation is a measure of the

proportion of soil bases to soil acids. • Bases = Ca, K, Mg, and Na • Acids = H, Al

• Most of the bases are essential for plant growth.

High CEC and High Base Saturation make for high productivity!

What Makes The Best Soil?• For

agriculture, the best type of soil is Loam.

Soil Concerns• Soil Degradation

• The loss of some or all of the ability of soils to support plant growth.

• Erosion remove topsoil which contains vital nutrients.

Main cause of degradation is soil erosion!

• Soil Compaction • Typically caused by machinery and humans.