Fallow, a portion of land left for a time without a crop to get rid of weeds, to render the soil more friable by exposing it to frost and drought, and to enable a fresh supply of soluble mineral substances, which may have been exhausted by previous crops, to be formed. This last object will be accomplished by water drawn up from below by capillary action; by substances brought down in small supplies by rain; by dust; by the action of earthworms; by atmospheric oxidation and hydration, and other spontaneous decompositions of previously insoluble mineral substances in the soil; and by nitrification (q.v.). Fallowing may be looked upon simply as a prolonged process of tillage. It differs in duration, "green crop fallow" extending from harvest to the time for sowing turnips and analogous crops in the following spring, and "summer," or "naked, fallow" continuing through the summer until the time for sowing autumn-sown wheat. Green crop fallowing begins with paring the stubble-fields directly they are reaped, and harrowing off the weeds, or using a "grubber," and leaving the weeds to be killed on the surface by winter frosts. Manuring is then carried out at once, and in the following spring the soil may be freed from weeds and loosened to the depth of 6 or 8 inches necessary for drilling green crops by the use of light grubbers. The longer "naked" fallow is now only used on very stiff clay soils. It consists in deep ploughing in autumn; a second ploughing and two cross-ploughings in spring; stirring by the grubber and harrow, so as to expose the weeds to drought, as often as possible in the summer; manuring in July; and adding lime, if necessary, before the final ploughing or "seed-furrow."