1249. When no artificial means are used, nature gradually restores the mineral substances which have been exhausted, and which the soil obtains from the atmosphere, the rain, and other sources. The restoration when left entirely to nature is slow, and thus it was the custom to let five or sometimes six years elapse before the same crop was grown twice on the same land, unless the process of restoration was aided by fallowing (see ï¾º 1217.) or by irrigation (see ï¾º 1237.).