What is parent material?

As humans, we typically have an idea who our mom and dad are. But where does soil come from?

Soil minerals are created by the gradual breakdown of large rocks into smaller and smaller pieces due to weather, chemical reactions, and interactions with living organisms such as plant roots and microbes. The original rocks are refered to as the soil's parent material. There are several major categories of parent material:

• Residuum (bedrock). Some soils are formed in place when the bedrock is weathered and broken down over time.
• Alluvium. This refers to sediments deposited by a river that develop into soil.
• Colluvium. Rocks falling due to gravity, for example down a mountainside, can accumulate and form a unique soil.
• Glacial till. During the ice ages, glaciers were powerful forces for shaping landscapes and rearanging materials.
• Loess and aeolian materials. These refer to the transportation of particles due to wind.

How does ancient parent material affect the soils we live on and work with today? In many cases, the properties of the parent material can directly explain characteristics seen in soil. For example, soils formed from limestone bedrock tend to inherit a moderate or high pH due to the chemical properties of calcium carbonate in their parent material. Colluvial soils tend to have a higher content of rocks within them. And alluvial soils are known for high fertility since they often contain nutrient-rich sediments. 

Yous soil has a parent (or multiple), just on a much longer timescale than human generations! What kind of heritage might your soil have?