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?