An image of rocky debris is shown next to an image of dark brown dirt.

Sam: What is the difference between sediment and soil? Sediments are deposits made up of broken pieces of bigger rocks, like sand or gravel.

Magnified many times, sand looks like tiny yellow rocks. From a regular distance we can see the many fine particles making up the sand.

Sam: Soil is special. Soil is a type of living deposit where water, plants, animals, and microorganisms all work together to change the rocks around them.

Several images flash by quickly. A puddle of water on top of soil. Green sprouts emerge from damp soil. A pink worm lays curled on top of some dirt. A microscopic images of soil microbes. A soil profile showing different soil horizons.

Sam: Soil forms from rock and we call this rock the parent material. Any type of rock can become a parent material. Parent material on the Earth's surface is exposed to water. This water can trickle through tiny cracks, opening spaces for plant roots to take hold. The water also changes the rock through chemical reactions that breaks it down into smaller pieces.

Animation. Water rains down on the parent material. The parent material starts to crack and weather. Small plants start to grow, their roots spreading down into the parent material. This creates the c horizon made of weathered rock.

Sam: As more rock is broken down larger plants start to grow on top. Dead plant matter falls to the ground and it makes a deposit called humus, a mix of weathered rock and organic matter.

The parent material break down further. Larger plants start to grow in it. Humus builds up on the top. The A horizon of the soil starts to form.

Sam: Over a very long time as water continues to seep down through the soil. The layers of weather rock just underneath the humus go through so many chemical changes that it becomes the simplest rock type, clay. The result is a soil with distinct layers or horizons.

It continues to rain. The water drips down through the A horizon and starts to form the B horizon. The B horizon is made up of clay. The A B and C horizons stack on top of each other creating a soil profile.