Crops

Crops, Rotation of, a principle of agricultural practice depending on the fact that the growth of one kind of plants, year after year, and the removal either of the entire produce or of the ripened fruit, rapidly impairs the general fertility of the soil. It is necessary, therefore, so to alternate different crops as to give the land an opportunity of acquiring fresh available supplies of those of its chemical constituents which have been removed in any crop before growing that crop upon it a second time. This replacement of waste may be effected by manuring, or by ploughing or digging in certain things, such as stubble or haulm, or by fallowing; but is most economically effected by rotation of crops. Whilst land is lying fallow, i.e. without a crop, or is under some other crop, rain, frost, sun, wind, dust, and worms either bring new matter to it, or loosen, pulverise, dissolve, or otherwise change substances in it, so as to render them available for the plant. It is usual to alternate grain crops and green crops, and, though the special circumstance of each soil and farm must be considered, what is known as the four-field or Norfolk system is generally adopted, by which turnips are followed by barley, barley half by clover and half by beans, peas, potatoes or vetches, and these by wheat or oats. Italian rye-grass may be substituted for clover, and at the next rotation the clover is sown on the half on which it was not before, so that clover comes only once in eight years on the same land. Similarly turnips may be replaced by mangel, carrots, cabbages, or swedes.