There is no known geological or soil science term called "placic horizon". It seems you might have misheard or misspelled a term.

Here are some possibilities and what they might relate to:

* Placic layer: This is a specific type of hardpan found in certain soils. It's a dense, cemented layer formed by the accumulation of iron and manganese oxides. Placic layers are often associated with poor drainage and can hinder root growth.

* Other possible soil horizons: Soil scientists use different terms to describe the different layers of soil, such as:

* O horizon: Organic matter layer

* A horizon: Topsoil

* B horizon: Subsoil

* C horizon: Parent material

* R horizon: Bedrock

If you can provide more context about where you encountered the term "placic horizon," I might be able to help you understand what it refers to.