SOIL AND SOIL FORMATIONSoil is a mixture of parent rock,decayed organic matter, minerals,water and air.
It takes thousands of years to form.
Factors affecting soilformation
Climate (Temperature and Humidity) Slope of land (topography) Type of parent material (rock type) Type of vegetation growing on the soil Time
Soil formation
1. Weatheringa) Chemical weatheringb) Mechanical weathering
2. Decomposition of organic matter (plantand animal remains)
Mechanical Weathering
Weathering breaks the rock into to smallerpieces called sediment. (sand, silt andclay)
Mechanical Weathering
Mechanical weathering includes: growingplants between rock, ice wedging andburrowing animals.
Temperature changes can break rocksapart.
Where is mechanical weathering mostimportant? Why?
Which biome(s) have themost important mechanicalweathering and why?
In temperate climates, winter is coldenough for water to turn into ice. Icecan crack rocks into little pieces
In the desert, the temperature changesgreatly each day. This cracks the rocks.
Chemical Weathering
The chemical composition of the rockschanges in chemical weathering becausechemical reactions occur.
Natural acids, such as carbonic acid in rainand plant acids cause chemical weathering.
Oxygen in the air causes oxidation.
Why is chemical weathering is important inwarm, humid climates?
Why is chemical weathering isimportant in warm, humidclimates? Chemical reactions are faster when the
temperature is higher.
Chemical reactions are faster in solution
Chemical weathering is very important inTropical climates.
Oxidationreactions oftropical soilcause it tobe reddishin color.
Properties of soil that changequickly Temperature
Moisture content
Composition of air in the soil pores
Properties of soil that changeover months or years Soil pH Soil color Soil structure Organic content in the soil Fertility of the soil Microorganisms Density
Properties that change overhundreds or thousands of years
Mineral composition of the soil Particle size in the soil Horizon formation
Soil Horizons
O horizon surface litter (leaves and twigs) A horizon topsoil layer (contains humus) B horizon subsoil (inorganic) C horizon bedrock and parent material E horizon forms when the conditions are
wet or forested (eluvial horizon) lighter incolor
R horizon: A rock layer that is sometimesunder the soil profile
Identify O, A, B, C and E
Soil profile from a temperateforest in Jordan
Soil Profile from Buenos Aires,Argentina
More about soil profiles
Sometimes, not all of the horizons will bepresent in some soils.
For example, O and E are only in forestedareas
Agricultural, deserts and grassy areas willstart with an A horizon but do not have anE horizon.
If the area was eroded, the A horizon maybe gone!
Shallow soils might not have the B horizon.
Soils around the world
Grassland soils Forest soils Tropical soils Cold climate soils (tundra) Arid soils Wet soils
Grassland Soils Common in Midwest
of the USA, Argentinaand Ukraine
Deep and dark Good for crops High organic content Holds water and
nutrients
Soil Profile fromPennsylvania, USA
Soil Profile from Buenos Aires,Argentina
Coniferous forest soils
The organicmatter comesfrom leavesand roots oftrees.
The acidsfrom the rainand leavesform acidswhich leachmaterialsfrom the tophorizons.
Tropical Soils The soil is deepand bright red(iron oxides)
The horizons arehard to tell apart
Organic matterdecomposes veryquickly andtransforms intoinactive materialthat binds to clay
Nutrients areleached from thesoils
Tropical Soil 3 hours fromBangkok, Thailand
Tropical Soil from Ghana
Tundra soils The soil is not
very developedand only faintindications ofhorizons canbe seen.
Permafrost canbe seen at thebottom of theprofile
Arid soil
The A horizon is light brown because thereis a lack of organic matter or moisture.
The organic matter is not returned to thesoil because there are not very many plants
When it rains, materials are carrieddownward into the profile to the B horizon
Calcium carbonate accumulates over timeand becomes hard
Wet soil
The A horizon is dark from accumulationof organic matter.
The soil is saturated and not enoughoxygen is present to decompose theorganic matter.
The B horizon is grayish.
Poor soil use leads todesertification
