Why does soil have different layers?

A cut through the ground can reveal dark soil near the top and lighter or differently colored material below. These layers did not all arrive as separate bags of soil. They developed as water, living things and weathering changed the materials already there. A side view through them is called a soil profile, and its layers are horizons.

Leaves and other remains collect near the surface. Organisms break them down and mix some of the material with mineral particles. Roots and small animals also change and mix the ground. In the illustrated profile, O contains organic remains and A mixes organic matter with minerals, which can make it darker.

Water carries some dissolved substances and fine particles downward. Material removed from one place can accumulate farther below, helping create a different horizon. In this example B contains material accumulated from above. Beneath it, C is partly changed parent material: the material from which this soil is developing. Weathering breaks or chemically alters minerals as the profile changes.

These additions, movements and changes explain why one patch of ground can contain different textures and colors with depth. Real soils do not all contain the same O/A/B/C stack, and their boundaries need not be sharp. A soil profile records a local history of living activity, water movement and changing mineral material.