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Name:______________________________ Period:________ Date:_________________
Soils
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2.5 Notes
Pages 56-67
Many organisms rely on soil for water, nutrients, shelter, or support. Soil is the loose material that covers the ground in most places. Most
soil is made up of a combination of sediment, water, air, and the decayed remains of organisms. However, soils are not all the same. They can
have different compositions and can form in different ways. A soil profile is broken down into horizons. The thicknesses of these horizons can
vary from place to place.
The horizon just below the Earth’s surface is called the topsoil. The topsoil is usually a fairly thick layer of dark, fine-grained, soft soil.
Topsoil contains plant roots and many living organisms. The topsoil also contains a component of soft, spongy, dark-colored material called
humus. Humus is the decayed remains of organisms. Humus contains most of the nutrients in the topsoil. It also helps the topsoil hold water.
This is the most fertile layer of the soil. A fertile soil contains the nutrients which help plants grow.
The horizon below the topsoil is called the subsoil. Subsoil layers are very thick and are composed of coarser-grains. These layers are
described as nutrient-poor and can be a light brown, gray, or reddish color. Very few organisms live in these layers.
If you were to dig down below the subsoil, you would find partially weathered bedrock and eventually solid bedrock. This bedrock is the
parent material from which the soil forms.
The texture of a soil is the relative amounts of gravel, sand, silt, and clay. Different soils have different ratios of these sediment types. Some
soils contain mostly clay. Others contain mostly sand. A soil that contains about equal amounts of clay, silt, and sand is called a loam. The
texture of a soil determines its porosity and permeability. The porosity of a soil is a measure of the amount of space between the solid particles
in the soil. The permeability of a soil is a measure of how easily and quickly water moves through it. Soils made up mostly of clay have a high
porosity and low permeability. Therefore, these soils hold a lot of water. Soils made up mostly of sand have a low porosity and hold very little
water. In other words, gravely and sandy soils drain better than soils that contain a lot of clay.
Soil Horizons
Porosity and Permeability
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Soils
Name:______________________________ Period:________ Date:_________________
P56-67
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