1169. All soils which abound in certain salts must be fertile; and this is the reason that some soils continue to produce crops year after year without manure. When the subsoil is of any hard rock, the productiveness of the soil is generally very greatly increased, as the rocky subsoil becomes gradually disintegrated, partly by mechanical and partly by chemical means, and thus continues to yield a constant supply of salts to the soil above it. Thus, a surface soil which has a granite subsoil is certain to be fertile, as the particles of granite disintegrate slowly; and a clayey subsoil will be of material advantage to a sandy soil, as the adhesive nature of the subsoil prevents the soluble salts in the soil from being washed away.
