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
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.
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.
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
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 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!
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.
Is Erosion Really a Problem?
Areas of serious concern
Areas of some concern
Stable or nonvegetative areas
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.