What is Soil?

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Subjects
Let's Talk Science
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5.8

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Learn about soil, soil texture and the various layers of soil.

Soil makes up the top layer of the Earth’s surface. It’s a mixture that includes minerals, water and air. Soil also contains organic matter like leaves, roots, twigs and bugs. 

Soil Texture


When you pick up a handful of soil, you can feel how coarse or fine it is. In other words, you can feel its texture
Soil mainly contains particles of sand, silt and clay. The ratio between these particles determines soil texture. Loam is soil that has equal parts sand, silt and clay.

Soil texture triangle showing the names for different types of soil. The type of soil is determined by the ratio of clay, silt and sand
Soil texture triangle showing the names for different types of soil. The type of soil is determined by the ratio of clay, silt and sand (Source: Let's Talk Science based on an image by Mikenorton [CC BY-SA 3.0] via Wikimedia Commons).
  • The diameter of sand particles ranges from 0.05 to 2.0 mm. This makes sand the largest type of soil particle. You can even feel individual grains with your fingers and see them with your eyes. Sand does not hold water or nutrients very well.
  • Silt particles are between 0.002 and 0.05 mm wide. You would need a magnifying glass to see individual grains. Silt feels like powder or flour. It holds very little water.
  • Clay particles are very small. You can’t see or feel individual grains. They have a diameter of less than 0.002 mm. Clay feels sticky when wet. It holds water and nutrients very well.

Did you know?

A gram of coarse sand contains about 1 000 particles. But a gram of clay contains about 90 billion particles! 

From left to right: Sand, silt and clay
From left to right: sand, silt and clay (Let’s Talk Science using image by: IDS.photos [CC BY-SA 2.0] via Wikimedia Commons, Robert Ashworth [CC BY 2.0] via Wikimedia Commons, and Marcus Cyron [CC BY-SA 3.0] via Wikimedia Commons).

Soil layers

Soil layers are called horizons. Most soil types have three or four different horizons.

Did you know?

It takes at least 100 years to form 2.5 cm of topsoil.

Based on their characteristics, soils horizons are named using different letters.

  • The O horizon covers the surface of the ground. It’s mainly composed of organic matter like decomposing leaves or bugs. This organic matter is called humus. Depending on the location, the O horizon can be very thick or entirely missing.
  • Often called topsoil, the A horizon consists of surface soil. This layer mainly contains minerals and decomposed organic material. Most plant roots grow in the A horizon.
  • The E horizon is also known as the eluviated layer. Clay, minerals, and organic matter leach down out of this layer. Sand and silt particles of quartz are left behind. It is often found in older soils and forest soils. 
  • The B horizon consists of subsoil, which lies beneath the surface soil. This layer has clay and mineral deposits. It has less organic material than the O and A horizons.
  • Also called the substratum, the C horizon contains very little organic material. Instead, it mainly consists of rock fragments. This rock is called parent material because soil develops from it. Plant roots do not grow in this layer.
  • The R horizon is also called bedrock. It consists of large areas of solid rock, including granite, limestone and sandstone.
Soil horizons
Soil horizons (Let’s Talk Science using an image by normaals via iStockphoto).

Misconception Alert

Some people think that soil is sterile, but it’s not. Soil actually contains billions of tiny living things. A teaspoon of soil can actually contain more microorganisms than there are people on Earth!]

Soil supports life on Earth in a number of ways. It plays a very important and complex role in different ecosystems.

Soil filters water. It supports buildings and roads. It gives crops and other plants a place to grow, while providing them with nutrients. Soil even helps clean the surrounding air and water. And the organisms that live in soil play an important part in the food cycles that make all life possible!

Did you know?

About 95% of human food production depends, either directly or indirectly, on the Earth’s soils.

Learn More

Layers Of Soil (2017)

Video (3:56 min.) from Peekaboo Kidz explaining the types of soil layers and their composition. Suitable for younger grades. 

What's the Dirt on ... Dirt? (2015)

Video (3:43 min.) from SciShow Kids discussing the composition of soil and the layers that make up soil. Suitable for a range of grade levels. 

Soil Testing: The Pioneer Way

Article by Anna Ikona for Homesteading discussing the importance of soil composition and quality, and how you can test the quality of soil at home. Includes infographics. 

References

Agriculture and Agri-Food Canada. (2014, December 15). Soil texture and water quality.

Soil 4 Kids. (n.d.). What is soil? Soil Science Society of America.

Soil Science Society of America. (n.d.). Soil texture.

Soil Science Society of America. (n.d.). Soil horizons.

University of Hawaii at Manoa. (n.d.). Soil composition.

University of Hawaii at Manoa. (n.d.). Soil texture and soil structure.