What ingredients make up soil?

Question: What ingredients make up soil?

Answer: Soils contain solid particles of different sizes and different types. Some of the particles are minerals, and others are organic. There are spaces between the particles; the spaces may be filled with air or water.


Photo courtesy of the Natural Resources Conservation Service.

Minerals in soil and how they “weather”

The mineral particles in soil form as rocks and minerals break into smaller pieces, or change into other minerals due to “weathering.” Water, ice, plant roots, heating and cooling of rocks, and rocks bumping into one another can break the rocks into smaller pieces. This is physical weathering: only the size has changed. The smaller particles are the same kind of rock or mineral as the larger particles from which they came.

Chemical weathering, in contrast, changes rocks or minerals into other rocks or minerals. The particles also may be smaller, but they are no longer like the rock or mineral from which they came.

Think about it this way. If you have a cracker and break it in half, each piece is still a cracker, just smaller. If you break it again, each piece is still a cracker. You might even be able to put it back together. This represents physical weathering.

Now put the cracker in your mouth and start chewing. Your teeth break the cracker into smaller and smaller pieces, but the saliva in your mouth mixes with the cracker as you chew. This starts the digestion process, and afterward the pieces of the cracker are no longer a cracker. You could not take the pieces out of your mouth and put them back together to form the cracker. This represents chemical weathering in which the rock or mineral is changed into another kind of rock or mineral.

Many people have used vinegar and baking soda to make a volcano. This is an example of chemical weathering. Baking soda is a mineral and vinegar is a weak acid. When they are mixed together, a chemical change occurs, water and carbon dioxide are released, and the solid that remains is no longer baking soda.


Humus is composed of the organic particles in the soil. These particles are the remains of plants, animals, and microorganisms after they have been decomposed by other organisms. Like you, organisms in the soil “eat” plant and animal tissues. Like you, they get energy from what they eat, and they produce waste. The waste produced by these soil organisms is called humus.

Particle size

Soil scientists separate the solid particles based on size. Particles larger than 2 millimeters (less than 1/10th of an inch) in size are gravel. Particles between 2 and 0.05 millimeters are sands. Particles between 0.05 and 0.002 millimeter are silts. Particles smaller than 0.002 millimeter in size are clays. The mixture of sand, silt, and clay particles determines soil texture. These individual particles form clumps, often held together by humus. Soil scientists call these clumps “peds.” The shape and size of these peds determines the soil structure.

So, soils are composed of mineral and organic solids, water, and air. Plants, animals, and microorganisms living in the soil affect the soil properties we observe, especially the amount of humus and the soil structure.

–Answered by Eric Brevik, Dickinson State University

Have a question for Soils Matter? Post it as a comment below, or email us at soils-matter@soils.org.

2 responses to “What ingredients make up soil?

  1. Pingback: Save the dates for the Spring 2015 Workshops at the Children’s Garden | UNA Children's Garden·

  2. Pingback: How was Great Sand Dunes National Park “made?” | Soils Matter, Get the Scoop!·

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