_Chemical effects of Water-Percolation._—The accumulation of plant-food in the subsoil is not, however, due only to the mechanically-carried particles, but also to the ingredients carried in solution from the surface soil and redeposited in the more retentive subsoils. Especially is this true of _lime carbonate_, which is dissolved by the carbonic acid formed chiefly within the humic surface soil, and is often carried off in amounts sufficient to obstruct drain tiles by its deposition in contact with air (see chapt. 3). In the case of moderate rains, however, it is carried no farther than the subsoil, and is there redeposited, in consequence of the penetration of air, following the water, and causing the carbonic gas to diffuse upward; thus leaving the lime carbonate behind. In the majority of cases this results simply in a gradual enriching of the subsoil in this substance; while the surface soil may become so depleted as to require its artificial replacement by liming or marling. The same general process occurs to a less extent, in the case of magnesia. _Calcareous Subsoils._—The fact that subsoils are more calcareous than the corresponding surface soils is often of great practical importance, in enabling the farmer to enrich his depleted surface soil in lime by subsoil plowing. The accumulation of lime carbonate in the subsoil also tends in a measure to offset the extreme heaviness sometimes resulting from the accumulation of clay. _Calcareous Subsoils and Hardpans._—When soils are very rich in lime, and rains occur in limited showers rather than continuously, the lime carbonate dissolved from the surface soil may accumulate in the subsoil so as to either form calcareous “hardpan” by the cementing of the subsoil mass; or it may accumulate and partly crystallize around certain centers and thus form white concretions, known to farmers as “white gravel.” The latter is the form usually assumed in the regions of summer rains; while in the arid regions the deficient rainfall causes this substance to accumulate, and calcareous hardpan to form, at definite depths depending upon the maximum penetration of the annual rainfall; sometimes in crystalline masses of veritable limestone (“kankar” of India), or sometimes merely as crystalline incrustations loosely cementing the subsoil.