CROPS 1536 



are actually injurious, to a second crop of the same species. 

 The debris of leguminous crops, e.g.^ clover, is particularly rich 

 in combined nitrogen, owing to the power which such crops 

 possess, of absorbing nitrogen from the air, and afford im- 

 portant supplies of this material, slowly (as nitrification occurs) 

 to a crop of, say, wheat, which follows. 



3. Variations in the relative requirements as to plant food. 

 Some crops require relatively large supplies of one particular 

 item of plant food. Thus an average crop of wheat or potatoes 

 consumes only about 50 lb. of nitrogen per acre, while an 

 average crop of mangolds removes about 150 lb. per acre. 



Again, too liberal a supply of nitrogen may do actual harm 

 to one crop, e.g.^ barley for malting purposes, but after the 

 addition of farmyard manure, barley may safely follow, if the 

 too abundant nitrogen be depleted by first taking off a crop of 

 some nitrogen-loving plant, e.g., mangolds. 



4. The obtaining of suitable mechanical conditions in the 

 soil. Some crops do best when the soil is loose and open at 

 seed time, e.5r.,bai ley, which often follows roots; the land is then 

 loose and friable and free from weeds, owing to the possi- 

 bility of hoeing during the growth of the turnips or mangolds. 

 Other seeds, e.g., wheat, grow best in a firm, compact soil, 

 and in this condition the land is left after a crop of clover, 

 which can be harvested early in the summer, giving ample time 

 for preparation of the land for the autumn sowing of wheat. 



Many systems of rotation are in vogue in difierent parts of 

 the country, various modifications being introduced to suit the 

 local conditions and requirements. For details of such systems, 

 reference to some manual of agriculture should be made. The 

 most generally adopted system is known as the Norfolk four- 

 course rotation, which in its simplest form, consists of — 



1. Roots, eaten on the land by sheep. 



2. Barley. 



3. Clover, generally made into hay, or sometimes grazed. 



4. Wheat. 



the farmyard manure being applied before the root crop. 



