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What is Soil

• Soil is a complex mixture of weathered rock, minerals, organic material (both living & decaying), water, and air.

• Different soils have different amounts.

• What might change the amounts?

• On average, soil has the following ratios:– 45% minerals & rocks

– 25% water

– 25% air

– 5% organic material

How is Soil Produced?

• Weathering of rocks and minerals

• Deposits of sediments washed/blown to the site

• Decomposition/actions of living things.

Weathering of Rock

• Weathering is the physical or chemical breakdown of rock into smaller pieces

• These pieces may be large or too small to see!

• Two types of weathering:

– Mechanical

– Chemical

Mechanical Weathering

• Physical breakdown of rock

• Chemical composition of the rock does not change

• Types:

– Exfoliation

– Ice Wedging

– Abrasion

– Plant & Animal Activity

Exfoliation

• When rocks come to the surface, pressure on them is reduced.

• This can allow cracks to form, and the rocks break apart.

• Example: Granite

Ice Wedging

• When water flows into the cracks in rocks & freezes.

• Why does this split the rocks?

Abrasion• When rocks broken from other processes

collide and break even more.

• Happens because of gravity, ice, running water, or wind…wait, how does wind do this?

Plant & Animal Activity• Plant roots act like ice to split rocks

• Animals, mostly ones that burrow, expose other rocks for further weathering.

Chemical Weathering

• Chemical reactions break the minerals in the rock into different materials

• Types:

– Oxidation

– Hydrolysis

– Carbonation

– Organic Acids

– Acid Rain

Oxidation• When elements

in the rock combine with Oxygen, and break off as new compounds.

• Usually happens in rocks with iron

• Produces a red color

Hydrolysis

• When chemicals in the rock react with water, causing some of the minerals to break away from the rock.

• Often happens with metals like Potassium & Aluminum

Carbonation• When CO2 gets into water it makes Carbonic Acid

• This acid can stick to some minerals and form compounds that are washed away by water.

• This often happens to limestone.

Organic Acids

• Some living things produce acids that are released to the environment.

• These acids can make cracks in rocks and start the weathering process.

• Examples: Lichens & mosses

Acid Rain

• Nitrogen & Phosphorus from fossil fuels get into the air and combine with rainwater

• This rainwater weathers rock more rapidly than normal rain.

Formation of Soil

• When new rock is exposed, soil begins to form.

• This newly exposed rock is known as Bedrock.

Formation of Soil• Weathering begins to break the bedrock into

smaller rocks.

• This layer of partially weathered rocks, above the bedrock, is known as Regolith.

Formation of Soil

• The top of the Regolith is weathered even more, producing small particles that begin to form soil.

• Sediments from water or wind may add to the soil.

• Also, organic material begins to add to the soil.

Formation of Soil

Mature soil

Young soil

Regolith

Immature soil

Bedrock

Soil Characteristics

• The main characteristics of a soil are determined by the bedrock it formed from…this is the Parent Material.

• Parent material can affect soil texture and soil color.

• Example: quartz-rich rocks produce sandy soil.

• Example: iron-rich rocks produce red soil.

Soil Characteristics

• The size of soil particles also affects the characteristics of a soil.

• Soils are usually categorized by the amounts of each type of particle that they have.

• Particles are:

– Clay: less than 0.0002mm

– Silt: 0.0002mm – 0.05mm

– Sand: 0.05mm – 2mm

Effects of Particle Sizes

• The size of soil particles affects how much water & air the soil can hold.

• The smaller the particles, the less the soil can hold.

• However, if the soil is made entirely of larger particles, the water will run right through the soil!

• We measure particle sizes by measuring Porosity.

Porosity:

The measure of the volume of pores and distance of pores

Effects of Particle Sizes

• Infiltration: the rate at which water from precipitation or other sources flows into the soil

Saturation:The amount of water and air

that can move through soilWater Water

High permeability Low permeability

Soil Profiles

• The formation of soil in different areas produces different layers.

• These layers are called Horizons.

• The composition and depth of horizons is measured by looking at a Soil Profile.

Soil Horizons

O horizon

Leaf litter

A horizon

Topsoil

B horizon

Subsoil

C horizon

Parent

material

Mature soil

Young soil

Regolith

Immature soil

Soil Horizons

• O Horizon:

• Top Layer/Surface litter layer

• Mostly plant debris (leaves, branches) and animal waste

• Brown/black in color

Soil Horizons

• A Horizon

• The Topsoil Layer

• A mixture of decomposing organic matter called Humus

• And…

• Small soil particles from the bedrock

• Contains most plant roots and soil organisms.

Soil Horizons

• B Horizon (Subsoil Layer)

• A transitional layer of parent material, organic material, and minerals leeched from the A horizon.

• Leaching is when water reacts with minerals in the upper soil layers, carrying them to the B horizon.

Soil Horizons

• C Horizon (parent material):

• This is the regolith…which is…?

• The partially weathered rocks from the bedrock.

• This layer usually has no organic material.

• Below the C horizon is the bedrock.

Other Soil Profiles

Different Soil Profiles in Rice Fields

What changes a Soil Profile?

• What determines the composition of a soil?

• Mostly the bedrock, erosion, & organic stuff…but what determines these?!?

• Climate!

• Climate mostly affects the layers of a soil, but not the materials found in the soil.

• In general, climates that have all 4 seasons have all the layers of soil.

Soil Layers & Climate

Tropical Rainforest Soils

• Large amounts of rain leech minerals from the upper layers to the B horizon, which becomes very thick.

• Almost no mechanical weathering…why?

• Decomposition is high…why?

• Would this be good for farming?

Tropical Soil Profile

Desert/Arctic Soils

• Very little rainfall, so almost no chemical weathering.

• Little life, so not much organic material.

• Soil is mostly regolith (C horizon) with a little organic material at the top (O horizon)

Desert Soil Profile

Temperate Soils

• This is where we live.

• Varying seasons provide all kinds of weathering.

• Lots of life, so there’s lots of organic material.

• Soils in these areas usually have all 4 layers.

Temperate Soil Profile

Soil Erosion

• What is erosion?

• The movement of weathered materials from one location to another.

• So…isn’t this the same as weathering?

• No! Weathering helps to build soil, soil erosion destroys soil.

What Causes Soil Erosion?

• Wind

• Gravity

• Water

• Glaciers

• Human Actions can speed up the process!

Wind Weathering

Wind Erosion

Gravity Weathering

Gravity Erosion

Water Weathering

Water Erosion

Glacier Weathering

Glacier Erosion

Humans & Erosion

• So, how do humans affect this Soil Erosion thing?

• Development for housing/buildings

• Farming…this is the big one

Humans & Erosion

• Farming can expose a lot of lose soil to erosion.

• This soil is the O and A horizons…why are these important?

• These contain most of the organic material in the soil, which plants and animals need to survive.

Farm Erosion

Is Erosion Really a Problem?

Areas of serious concern

Areas of some concern

Stable or nonvegetative areas

What can we do to prevent Erosion?

Contour Plowing

• Hillsides are plowed in curves that follow the shape of the land.

• Prevents water from flowing straight down.

Strip Cropping

• Crops are planted in alternating bands.

• This way, the whole area is not filled with row crops that don’t cover all the land.

Terracing

• Hills are carved into flat “steps”.

• This prevents water from flowing rapidly down the hill.

Crop Rotation

• Planting a crop to harvest 1 year, and one to leave in place the next year.

• Often combined with strip and contour farming.

Quick Quiz #2

• In your experiments, the variable you measured is known as the _______________ variable.

• This region would have a large B horizon in the soil, full of minerals leeched from the upper layers.

• These soil particles are the smallest.

• The measure of the rate of water movement into the soil.