1213. The depth of pulverisation must depend upon the nature of the soil and of the subsoil, and also on the nature of the plants grown in the soil. In orchards and kitchen gardens where there are fruit trees, deep pulverisation is exceedingly injurious, as it prevents the spongioles of the roots from rising to the surface of the soil, and consequently places them beyond the reach of atmospheric air. Annual crops, on the contrary, are generally better when the soil which is to receive them has been prepared by tolerably deep pulverisation. Care must, however, always be taken not to dig below the surface soil, as sometimes, by very deep trenching, the surface soil may be buried, and the subsoil brought to the surface, which, if the subsoil chances to be sand or gravel, will be a change decidedly unfavourable. In some cases, however, a mixture of the subsoil with the surface soil is decidedly beneficial, as it may supply the surface soil with earthy or mineral substances which may have become exhausted.