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Author(s): Nancy Moreno, PhD, and Barbara Tharp, MS.

What Is Soil Made Of?

Carbon, oxygen and hydrogen are the building blocks of the molecules that make up our bodies, our foods and even the fuels we burn. These elements are combined during photosynthesis to make energy-rich materials, such as sugars and other carbohydrates (starches). Plants obtain hydrogen from liquid water (H2O). They obtain carbon from carbon dioxide (CO2) gas in air. Oxygen is part of both water and carbon dioxide, and is present as oxygen gas (O2) in air. However, all living things, including plants, require additional materials to carry out the chemical processes necessary for life.

Where do these other materials come from? Most of them are released into water from soil. Plants and plant-like organisms, such as algae, absorb nutrients dissolved in water. Examples of these nutrients include nitrogen, phosphorous and potassium. Non-photosynthetic organisms obtain the minerals and more complex molecules that they need by consuming plants and other living things. Thus, the nutrients in soil are important not only for supporting plant growth, but also for assuring that other organisms are able to grow and survive.

Soil has both living and non-living components. It constantly changes through the action of weather, water and organisms. Soil formation takes a very long time—up to 20,000 years to make 2.5 cm of topsoil! This is only as deep as a quarter standing on its side!

The non-living parts of soil originated as rocks in the Earth’s crust. Over time, wind, water, intense heat or cold, and chemicals gradually break rocks into smaller pieces, a process known as weathering. The size and mineral composition of the tiny rock particles determine many of the properties of soil.

Most soils are enriched by decomposed plant and animal material. Soil is home to many kinds of organisms: bacteria; fungi; algae (plant-like organisms that live in water or moist environments); earthworms; insect larvae; and plant roots, to name a few. Soil also contains many tiny air spaces. Typical garden soil is 25% water, 45% minerals, 5% material from living organisms and 25% air.


Funded by the following grant(s)

National Institute of Environmental Health Sciences, NIH

National Institute of Environmental Health Sciences, NIH

My Health My World: National Dissemination
Grant Number: 5R25ES009259
The Environment as a Context for Opportunities in Schools
Grant Number: 5R25ES010698, R25ES06932


Houston Endowment Inc.

Foundations for the Future: Capitalizing on Technology to Promote Equity, Access and Quality in Elementary Science Education