What is a Soil?

Soils are the critical outer carapice of Earth’s surface, where water, life, rock, and energy meet and form the basis for the world that we know.

Learning Goals

• Define soil and mobile regolith
• Build a basic background and vocabulary on soils

Acknowledgment

Some of this material is from and/or inspired by Prof. Phil Larson’s Physical Geography course.

Reading

What are soils? (From Nature)

This introduction to soils from the US Department of Agriculture’s Natural Resources Conservation Service (NRCS) takes you through a whole set of definitions surrounding soils. The information here will overlap many of the lessons in this section, and reading it will give you a roadmap that should speed your way through understanding and describing the basics of soils.

Video

This MinuteEarth piece is a good follow-on from Dave Montgomery’s Nobel Conference lecture by giving some general scientific information about soils, soil erosion, and agricultural sustainability. Note the animated expansion of fluvial erosional networks! It’s a teaser for linking soils, hillslopes, and channel systems.

Definitions

Soil

“Layered residue of decomposed rock and organic matter left by weathering over an extended period of time.” (Bierman and Montgomery, 2014)

“(i) The unconsolidated mineral or organic material on the immediate surface of the Earth that serves as a natural medium for the growth of land plants. (ii) The unconsolidated mineral or organic matter on the surface of the Earth that has been subjected to and shows effects of genetic and environmental factors of: climate (including water and temperature effects), and macro- and microorganisms, conditioned by relief, acting on parent material over a period of time. A product-soil differs from the material from which it is derived in many physical, chemical, biological, and morphological properties and characteristics.” (USDA Natural Resources Conservation Service)

Regolith and Parent material

Regolith is the broken-down form of the original material – often bedrock – present in a landscape or at the base of a soil profile.

Parent Material is the original material from which the regolith is formed. It can be bedrock, wind-blown sediment, glacial till, etc.

Mobile Regolith is exactly what it sounds like: regolith that can move! In geomorphology, sometimes we are concerned with soil formation, and sometimes we just want to know what is physically attached to the hillside vs. what can be easily mobilized and transported.

Soils Geomorphology

Soils geomorphologists use soil formation to understand how a landscape forms and changes and to provide relative dates on landforms. They often investigate past climate, erosion, and depositoin in the context of soils.

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