What Is the Difference Between Soil and Regolith?

While regolith is a layer of loose, uncompacted dirt, dust and rocks sitting on top of bedrock, soil is the portion of the regolith that is able to support plant life. Soil contains organic matter, liquids and minerals, while most of the other layers of the regolith do not.

Regolith appears not only on Earth but also on the Moon, Mars and Saturn’s moon Titan. Soil, however, has only been found on Earth, the only one of those bodies to support plant life. The regolith on Earth also contains alluvium, weatherized rocks, volcanic lava and ash, clay, groundwater and salt. The word “regolith” comes from the Greek for “blanket rock,” and indeed the regolith is a version of rock that blankets the earth.

Typically, people refer to dirt that contains organic material as soil, while referring to other elements of the regolith as dirt, dust, gravel or sand. There is no uniformity to the dispersion of soil across the Earth. It can be completely absent, as on a sandy beach or exposed bedrock, or it can cover the Earth’s surface at a depth of 30 or more feet. Regolith is created from bedrock through many natural means, including weathering, exposure to water or chemicals that break down the rock, thermal expansion or destruction of bedrock by the intrusion of plant roots.