The Nature of SoilThe Nature of Soil
Essential Question: What are the characteristics of soil? How does the composition
of soil vary?
What is Soil?What is Soil?Soil is the loose, weathered material
on Earth’s s surface in which plants can grow
Soil is created from broken down bedrock
Bedrock is the solid layer of rock beneath the soil
When this rock is exposed, it gets weathered or broken down into small particles that forms soil
What is soil made up of?What is soil made up of?Soil is made up of:
◦ Rock/mineral fragments◦ Decayed Organic matter◦ Air◦ Water
Rock/Mineral fragments are broken down into small particles such as clay, silt, and sand
Most decayed organic material comes from plants, but also from animals when they die
Eventually, this turns into humus, a dark-colored substance that forms as plants and animals decay
Humus contains nutrients that plants need to help them grow
Good soil has an equal mix of humus and weathered rock
Different types of Soil:Different types of Soil:Soil texture (how it feels) can vary
depending on the particle size in the soilSand feels coarse and grainy, while clay
feels smooth and silkySoil texture is important for plant growth:
◦Sandy soils allow water to quickly drain, so plants may not get enough water
◦Clay soils hold a lot of water, so plants may drown from too much water
Good soil should have equal parts of sand, silt, and clay
How does soil form?How does soil form?Soil forms as rock is broken down by
weathering and mixes with other material on the surface.
Soil is constantly being formed whenever bedrock is exposed and weathered.
Plants and animals add organic matter to rock fragments
Organic Matter is the remains of once living organisms (plants and animals)
What is a soil profile?What is a soil profile?As soil develops it forms layers
known as horizonsHorizons are the different layers
of soil that differs in color and texture from the layer above or below it
All of the horizons together form a soil profile
What is a Soil Profile?What is a Soil Profile?
A Horizon: the top layer of soil, usually covered with litter, or leaves, twigs, and
other organic material
B Horizon: the subsoil layer. Lighter in color due to less humus and is less fertile.
Bedrock: solid rock below soil
O Horizon: layer of loose leaves and organic debris at the surface of the soil
C Horizon: the parent rock. Mostly weathered rock and is bottom of the soil
profile. It is most like the bedrock material.
What is Leaching?What is Leaching?Leaching is the
removal of minerals that have been dissolved in water
When it rains, the water picks up minerals and carries them down from the A Horizon to the B Horizon.
C Horizon will hardly ever have minerals in it
How do living organisms How do living organisms affect soil?affect soil?
Some organisms make humus, the material that makes soil fertile
Other organisms mix the soil and make space for water and air
Plants contribute most of the organic remains that form humus◦Litter is the loose layer of dead plant leaves
and stems of the soils surfaceDecomposers are the organisms that
break down the remains of dead organisms (fungi, bacteria, worms, etc)
Factors affecting soil formation include:
◦Climate
◦Slope of land
◦Types of Rock
◦Types of Vegetation
◦How long the rock has been weathered
What factors affect soil?What factors affect soil?
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