xii. CROPS 259 



abundance of one constituent, e.g., nitrogen, may actually dimmish the 

 value of a crop, e.g., barley for malting, though another crop say 

 mangolds would be greatly benefited by such liberal supplies of 

 nitrogen. 



The table on the following page gives, according to Warington's 

 estimates, the weight, in pounds, of the chief substances removed from 

 an acre of land by an average crop of the various plants. 



Inspection of the figures will show that the demands of different 

 crops upon the soil for nitrogen, lime, phosphoric acid and potash 

 are very different. A crop of mangolds, for example, removes about 

 three times as much nitrogen, nearly ten times as much potash and 

 more than twice as much phosphoric acid as a crop of cereals. So, 

 too, while a supply of about 50 Ib. of nitrogen per acre will satisfy the 

 requirements of cereals or potatoes, twice that quantity is required by 

 a crop of turnips, swedes, beans, or clover. In the case of the last two 

 crops, however, this is not necessarily derived from the soil, since 

 leguminosce have their own supply of nitrogen and, as a matter of 

 fact, it is well known that the land is left richer in nitrogen after such 

 a crop has been removed. 



Crop residues roots and leaves left in the soil after the removal 

 of a crop, afford suitable food for another crop of a different species, 

 but do not serve nearly so well for a second crop of the same species. 

 Moreover, insect pests and diseases, to which a crop is liable, are 

 very likely to be carried on from year to year if the same crop be grown 

 in succession, but tend to die out if the affected crop be followed by 

 others upon which the pest cannot prey. 



Sometimes, several years are required before the land becomes free 

 from the germs of a particular disease-causing organism, but crops 

 which are not subject to the disease can be grown successfully in the 

 interim. "Clover sickness," to which certain land is subject, affords a 

 good example, and " finger-and-toe " in turnips is another. 



Still another great advantage of a system of rotation is the oppor- 

 tunities it affords of periodically freeing the land from weeds. While 

 root crops occupy the land, hoeing between the drills can be made 

 nearly as effective in getting rid of weeds as the less profitable practice 

 of " bare fallowing ". 



Many systems of rotation are practised in different districts. One 

 of the favourite ones is that known as the Norfolk rotation, which, in 

 its simplest form, consists in 



1. Turnips or swedes, or mangolds or potatoes. 



2. Barley. 



3. Clover, or beans or peas. 



4. Wheat. 



Various modifications are introduced into this system to suit parti- 

 cular circumstances. Barley may be replaced by oats or even by 

 wheat, and the clover or "small seeds" often clover and ryegrass 

 may be left on the land for two years. Where clover sickness is pre- 

 valent, beans, peas, or vetches are grown every eighth year instead of 

 .clover, and where finger-and-toe exists, mangolds or potatoes replace 

 Jburnips alternately. 



