Hardpan is a common name for a compaction layer below ground that can be caused from plowing, driving over the area, etc. Basically it is as the name implies just like… a "hard pan". (MORE)

It's not 100% impermeable, but it is hard enough that it allows very little water to pass through. "Hardpan" is a word farmers use to indicate a layer of very compacted soil b…eneath the topsoil layer, frequently caused by large farm equipment, and is otherwise referred to as "plowpan." It is frequently a hard enough layer that crop roots are unable to pass through as they grow, even though meristematic (root tip) tissue can exert hundreds of pounds per square inch of force to try to break through. If the crops are unable to reach water and nutrients due to relatively impermeable soil, the crop will not be as vigorous or yield as well as it could. (MORE)