Forage Crops.The mangold-wurzel, occupying four times the acreage of swedes and turnips, is by far the chief root-crop in France.
The cleaning process is carried on through the next summer by means of successive hoeings of the spring-sown root-crop. As turnips or swedes May occupy the ground till after Christmas little time is left for the preparation of a seed-bed for barley, but as the latter is a shallow-rooted crop only surface-stirring is required.
They are effected chiefly by some alteration in the description of the root-crop, and perhaps by the introduction of the potato crop; by growing a different cereal, or it may be more than one cereal consecutively; by the growth of some other leguminous crop than clover, since " clover-sickness " may result if that crop is grown at too short intervals, or the intermixture of grass seeds with the clover, and perhaps by the extension by one or more years of the period allotted to this member of the rotation.
The greater part of the nitrogen of the cereals is, however, sold off the farm; but perhaps not more than to or 15% of that of either the root-crop or the clover (or other forage leguminous crop) is sold off in animal increase or in milk.
Over and above the great advantage arising from the opportunity which the growth of root-crops affords for the cleaning of the land, the benefits of growing the root-crop in rotation are due (1) to the large amount of manure applied for its growth, (2) to the large residue of the manure left in the soil for future crops, (3) to the large amount of matter at once returned as manure again in the leaves, (4) to the large amount of food produced, and (5) to the small proportion of the most important manurial constituents of the roots which is retained by store or fattening animals consuming them, the rest returning as manure again; though, when the roots are consumed for the production of milk, a much larger proportion of the constituents is lost to the manure.
