Rock, remains and life
What Is Dirt Actually Made Of?
Soil is crushed rock mixed with rotted remains, air, water and an enormous amount of living things.
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Step 1 of 5
Most of it began as solid rock.
Frost, roots, rain and rivers break rock into ever smaller pieces over thousands of years, and those fragments form soil's skeleton.
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Step 2 of 5
Grain size decides how soil behaves.
Sand grains are large with big gaps that drain fast. Clay particles are minute, hold water tightly and pack together almost solidly.
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Step 3 of 5
Dead things make up the rest.
Fallen leaves, roots and animal remains rot down into a dark spongy material that holds water and carries most of soil's nutrients.
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Step 4 of 5
Half of good soil is empty space.
The gaps between particles hold air and water, and roots need both. Compacted soil with no gaps is where plants struggle most.
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Step 5 of 5
And it is extraordinarily alive.
A single handful holds billions of bacteria and fungi doing the rotting, and their threads physically hold the soil together.
One teaspoon of healthy soil can hold more microorganisms than there are people on Earth.
The short version
Half of good soil is empty space, which is where roots find their air and water. Grain size decides the rest — sand drains, clay packs almost solid — and one handful holds billions of bacteria and fungi whose threads hold it together.
Try it yourself
Shake a jar of soil with water and let it settle. It separates into sand at the bottom, then silt, then clay, with debris floating.