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What Is Soil Made Of?

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

The Science of Dirt

Soil is a combination of many different living and non-living things. Soil contains many kinds of organisms: bacteria, single-celled organisms, fungi, algae, decomposed animal and plant material, plant roots, insects, and other animals, as well as air spaces and water. Soil develops very slowly over time from weathered rock and sand. The non-living parts of soil originate as rocks in the Earth’s crust. Over time, wind, water, intense heat or cold, and chemicals gradually break rocks into smaller pieces. The size and mineral composition of these tiny rock particles determine many properties of soil. Typical garden soil is 25% water, 45% minerals, 5% material from living organisms and 25% air.

Soil provides raw materials needed by all living things. 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 are released into water from soil. Plants and plantlike organisms, such as algae, absorb nutrients that are dissolved in water, such as nitrogen, phosphorous and potassium. 

Non-photosynthetic organisms obtain the minerals and more complex molecules they need by consuming plants and other living things. Thus, the nutrients in soil are needed not only to support plant growth, but also to enable other organisms to grow and survive. 

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