Encyclopedia of Science | Soil

• Words to Know
• Soil horizons and profile
• Life in the soil
• Soil erosion

Soil, which covers most of the land surface of Earth, is a complex mixture of weathered rock debris and partially decayed organic (plant and animal) matter. Soil not only supports a huge number of organisms below its surface—bacteria, fungi, worms, insects, and small mammals—but it is essential to all life on the planet. Soil provides a medium in which plants can grow, supporting their roots and providing them with water, oxygen, and other nutrients for growth.

Soil now covers Earth in depths from a few inches to several feet. Soils began to form billions of years ago as rain washed minerals out of the molten rocks that were cooling on the planet's surface. The rains leached or dissolved potassium,...