Where does soil come from?

What does soil have to do with a nature blog? Well, without dirt there simply could be no land-based life on earth. Soil has been called "the bridge between life and the inanimate world" because it contains many essential elements needed by living things.

Soil is about 45% mineral matter (chemical elements) that comes from weathered and decomposed bedrock, 5% organic matter that comes from decayed plant and animal life (humus), 25% air, and 25% water.

Essential elements needed by life are released from minerals contained in rock and made available to the soil through processes of mechanical and chemical weathering.

These are the 15 essential elements required by all plants and animals (1):

- Hydrogen(H)
- Carbon (C)
- Nitrogen (N)
- Oxygen (O)
- Sodium (Na)
- Magnesium(Mg)
- Phosphorus(P)
- Sulfur(S)
- Chlorine (C)
- Potassium (K)
- Calcium (Ca)
- Manganese (Mn)
- Iron (Fe)
- Copper (Cu)
- Zinc (Zn)
- Selenium (Se).

These 8 elements (in order of abundance) make up 98% of the earths crust (2):

- Oxygen (O)
- Silicon (Si)
- Aluminum (Al)
- Iron (Fe)
- Calcium (Ca)
- Sodium (Na)
- Potassium (K)
- Magnesium (Mg)

All other elements amount to only 1.5% of the crust.

Water is an important mechanical weathering agent for rock. When water freezes its volume increases by 9%. Any water that seeps into the cracks of rock and freezes exerts tremendous pressure that wedges and splits the larger parent rock into smaller and smaller fragments.

Since these smaller fragments have a greater surface area than the original parent rock, that makes them more vulnerable to a second important weathering process, chemical weathering. And water is important here, too. Rain picks up carbon dioxide (CO2) from the atmosphere as it falls and thereby becomes weakly acidic (carbonic acid). The acidic rain gradually causes the chemical decomposition of most rock minerals into solution.

Water mixes with essential elements in the soil that were released from rock minerals by mechanical and chemical weathering. The enriched water solution is then absorbed by plants, which in turn introduce the essential elements into the food chain.

For example, calcium and phosphorous are essential elements for bone. Calcium is abundant in nature, but phosphorous is rare. As granite decomposes through mechanical and chemical weathering, so does the embedded mineral apatite, which contains the essential element phosphorous. Plants absorb the phosphorous when it is released into the soil, and they in turn are eaten by animals.

This is an vast oversimplification of a very complicated subject, and probably way more than anybody wants to know about soil. But the more I learn about our natural world, the more fascinating it becomes.

(1) Source:
http://www.usgs.gov/faq/faq.asp?id=248&category_id=29

(2) Source:
http://www.windows.ucar.edu/tour/link=/earth/geology/crust_elements.html