Soil is not just dirt. It is a mixture of weathered rock, water, air and humus, the dark matter left by dead plants and animals. A healthy soil is roughly half solid (mostly rock grains, with a little humus) and half pore space filled with water and air.
- O horizon: fallen leaves and other organic matter on top.
- A horizon (topsoil): dark, rich in humus, full of roots and living things.
- B horizon (subsoil): lighter, with clay and minerals washed down from above.
- C horizon: broken, weathered pieces of the rock below.
- R: solid bedrock.
Key idea
Soil forms slowly, from the top down, as bedrock weathers and living things add organic matter. The layers that form are called horizons: O, A, B, C, then bedrock.
Why it works
Weathering is fastest near the surface, where water, air and roots reach, so rock breaks down there first and the soil deepens over time. Plants grow, die and decay, adding humus at the top, which makes topsoil dark. Rainwater soaking down carries clay and dissolved minerals from the top layer into the next, building the subsoil. That is why horizons stack in the same order nearly everywhere.
Worked example: Air in the soil
A 2,000 cm³ scoop of garden soil is about 25% air. How much air is in it?
- 25% is one quarter.
- $2{,}000 \times 0.25 = 500$ cm³ of air.
Answer: 500 cm³
Worked example: Reading a soil pit
In a soil pit you see, from the top: dark crumbly soil full of roots, then reddish clay-rich soil, then broken chunks of rock. Name them.
- Dark, with humus and roots: A horizon (topsoil).
- Clay and minerals washed down: B horizon (subsoil).
- Broken rock: C horizon. Bedrock would be below.
Answer: A, B, then C
Common mistake
Thinking soil is just dirt that was always there. Soil forms from rock, water, air and living things over long times; in many places a few centimeters of topsoil took centuries or more to build. That is why losing topsoil to erosion is hard to undo. Check: the bottom of a soil pit almost always shows broken rock, the soil's parent material.