The bottom zone of a soil horizon is called the R horizon, also known as bedrock.

Here's why:

* R horizon represents the unweathered parent material. It's the solid, unfragmented rock that lies beneath the soil layers.

* Bedrock is the foundation on which soil develops. It's the source of the minerals and particles that make up the soil above.

It's important to note that the R horizon is not always present. In some cases, the soil profile may end with a layer of weathered bedrock, known as the C horizon.

Here's a quick summary of the soil horizons:

* O horizon: Organic matter (dead leaves, twigs, etc.)

* A horizon: Topsoil (dark, rich in organic matter)

* E horizon: Zone of leaching (lighter color, minerals washed out)

* B horizon: Subsoil (accumulation of minerals)

* C horizon: Weathered bedrock (broken down rock)

* R horizon: Bedrock (solid, unweathered rock)