compacted soil layer nearly impervious to water. air. and roots.

Merriam-Webster Online Dictionary
hardpan (noun)
a cemented or compacted and often clayey layer in soil that is impenetrable by roots
a fundamental part - bedrock
hardpan (Wikipedia)
For the geography term, a dry terminus of an internally drained basin in a dry climate (i.e. salt flat/playa), see Dry lake.

In soil science, agriculture and gardening, hardpan or soil pan is a dense layer of soil, usually found below the uppermost topsoil layer. There are different types of hardpan, all sharing the general characteristic of being a distinct soil layer that is largely impervious to water. Some hardpans are formed by deposits in the soil that fuse and bind the soil particles. These deposits can range from dissolved silica to matrices formed from iron oxides and calcium carbonate. Others are man-made, such as hardpan formed by compaction from repeated plowing, particularly with moldboard plows, or by heavy traffic or pollution.

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