“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
Even though soil is only a very thin layer on Earth’s surface over the solid rocks below, it is the where the atmosphere, hydrosphere, biosphere, and lithosphere meet. We should appreciate soil more.
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 that 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
Soil is an ecosystem unto itself. In the spaces of soil, there are thousands or even millions of living organisms. Those organisms could include earthworms, ants, bacteria, or fungi (Figure below).
Earthworms and insects are important residents of soils.
inorganic: Not organic; not involving life or living organisms. For example, the rock and mineral portion of the soil.
loam: Soil texture that forms from a roughly equal combination of sand, silt and clay.
organic: Something from living organisms.
permeable: A material with interconnecting holes so that water can move through it easily.
soil: The top layer of Earth's surface containing weathered rocks and minerals and organic material.
- 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.
Use this resource to answer the questions that follow.
1. Why is soil important?
2. How many different types of soils are there?
3. Explain the composition of average soil.
4. What is humus?
5. What does the amount of humus determine?
6. What is texture?
7. How can texture effect plant growth?
8. What type of soil do farmers prefer?
9. How is soil being lost each year?
10. List the different types of erosion.
1. What is the inorganic material that makes up a soil?
2. What is the organic material that makes up a soil?
3. If a soil has equal amounts of silt, clay, and sand, what type of soil is it?