The layer of earth that sits just under the topsoil — under the dark, soft dirt where plants have their roots, but above the solid rock deeper down. Topsoil is the part that is full of dead leaves and old plant matter, so it is dark and loose and alive; subsoil is what you hit when you keep digging. It is usually paler, harder, and more full of clay, sand or small stones, because the good dark stuff has not been mixed into it. Plants can push roots down into it and it holds water, but little grows in it directly. The word is mostly used by farmers, gardeners, builders and geography teachers rather than in daily talk — if you are digging a hole for a house or a swimming pool, the subsoil is what you spend most of your time digging through. Note that people are a little loose about where topsoil ends and subsoil begins; there is no sharp line under the ground, and the two shade into each other. It is nearly always a noun and nearly always uncountable — 'heavy subsoil', 'the subsoil is clay'. There is also a rarer farming verb, to subsoil a field, meaning to break up that hard lower layer with a deep blade so water and roots can get through it; you will meet that mostly in farming writing.

心土、下層土（農業上也說「底土」）