Regolith

Unless you are using hydroponics, all of your plants will grow in some sort of soil. The better we understand our soil, the easier it will be for us to take good care of it (and easier is good).

Soil forms layers (Public Domain)

Soil tends to form layers on top of bedrock. These layers of soil are called horizons. Just above the bedrock is a layer of unbroken rock. There are generally no plant roots here. This is called the regolith. 

There are four basic soil horizons:

• O horizon - very thin, dark top layer made up of decomposed organic material
• A horizon - topsoil where most plant roots are found, which is made up of decomposed organic material and minerals
• B horizon - lighter subsoil, with more minerals and less organic material
• C horizon - regolith layer of unbroken rocks

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All of this normally sits on top of a hard layer of solid bedrock. The U.S. Department of Agriculture has an amazing interactive map that can help you learn more about your local soil.

​Artificially and naturally occurring layers of impenetrable hardpan can thwart even the best gardeners.

The best way to understand your soil is with a lab-based soil test.