“Land, then, is not merely soil; it is a fountain of energy flowing through a circuit of soils, plants, and animals.” — Aldo Leopold, A Sand County Almanac, 1949
Characteristics of Soil
Soil is a complex mixture of different materials.
- About half of most soils are inorganic materials, such as the products of weathered rock, including pebbles, sand, silt, and clay particles.
- About half of all soils are organic materials, formed from the partial breakdown and decomposition of plants and animals. The organic materials are necessary for a soil to be fertile. The organic portion provides the nutrients, such as nitrogen, needed for strong plant growth.
- In between the solid pieces, there are tiny spaces filled with air and water.
Within the soil layer, important reactions between solid rock, liquid water, air, and living things take place.
In some soils, the organic portion could be missing, as in desert sand. Or a soil could be completely organic, such as the materials that make up peat in a bog or swamp (Figure below).
Peat is so rich in organic material, it can be burned for energy.
The inorganic portion of soil is made of many different size particles, and these different size particles are present in different proportions. The combination of these two factors determines some of the properties of the soil.
- A permeable soil allows water to flow through it easily because the spaces between the inorganic particles are large and well connected. Sandy or silty soils are considered "light" soils because they are permeable, water-draining types of soils.
- Soils that have lots of very small spaces are water-holding soils. For example, when clay is present in a soil, the soil is heavier, holds together more tightly, and holds water.
- When a soil contains a mixture of grain sizes, the soil is called a loam (Figure below).
A loam field.
When soil scientists want to precisely determine soil type, they measure the percentage of sand, silt, and clay. They plot this information on a triangular diagram, with each size particle at one corner (Figure below). The soil type can then be determined from the location on the diagram. At the top, a soil would be clay; at the left corner, it would be sand; at the right corner, it would be silt. Soils in the lower middle with less than 50% clay are loams.
Soil types by particle size.
Soil, the Ecosystem
Earthworms and insects are important residents of soils.
- Soil reflects the interactions between the lithosphere, atmosphere, hydrosphere and biosphere.
- Permeable soils allow water to flow through.
- The proportions of silt, clay, and sand allow scientists to classify soil type.
- What is the inorganic material that makes up a soil?
- What is the organic material that makes up a soil?
- If a soil has equal amounts of silt, clay, and sand, what type of soil is it?
Use this resource to answer the questions that follow.
- What is soil? Does it contain organic or inorganic materials?
- Why is soil a habitat?
- What does soil do for water?
- Where does the organic material in soil come from?
- Where does the inorganic material come from?
- What happens as a soil matures?
- Why are mature soils best for plant growth?
- What determines the characteristics that a soil will ultimately have?
- What is the mnemonic device for the layers of soil: OAEBC?
- What is in the O layer?'
- What makes good topsoil?
- Where is the E zone?
- What does the E stand for? What happens in the E zone?
- What is zone B?
- What is zone C?
- What important feature does particle size determine? What does that mean?
- What makes up the most porous soil type? The middle soil type? The least porous soil type?
- How is soil classified?
- What about clay is important for plants?
- What is 90% of the biological stuff in soil? What is the remaining 10%?
- What happens when a soil is degraded?
- Which soils are most vulnerable to erosion?