CHAP. VIII ROTATION OF CROPS 63 



acid ; and others, again, want a good deal of lime and mag- 

 nesia, and less of the other inorganic elements. For instance, 

 turnips take up from the soil five times as much potash as 

 wheat ; barley takes up twenty-six times as much silica as 

 wheat ; and clover requires eight times as much lime and 

 magnesia as wheat, and only one-sixteenth the quantity of 

 silica. If, therefore, turnips be grown on the land one year, 

 and barley, clover, and wheat be grown on the same land for 

 three succeeding years, it is found that the soil in the four 

 years gives a much greater return, with less injury to the 

 land than if one of the crops were raised every year for 

 four years. This is the system usually employed in Norfolk Norfolk 

 in England, and it is called the Norfolk course of rotation. roStbn° 

 In some parts of the world the soil is so deep and rich that, 

 with proper tillage, the same crops can be grown on it for 

 numbers of years, without the application of any manure ; 

 but, even with this extra fertile soil, the time comes when 

 exhaustion takes place, and in certain districts of the United 

 States of America the land which was once exceedingly 

 fertile has been almost ruined by constantly taking off the 

 same crops. It is a principle in rotation that two grain 

 crops should not succeed each other, but be separated by 

 root or fodder crops. Thus it would be a very bad system 

 to take maize crops off the same land for two years running ; Wesf f ndfa^n 

 but, if tanias or yams be planted the first year, maize the course of 



' •' ^ -^ ' rotation, 



second, sweet potatoes the third, and castor oil or some such 

 crop the fourth, a very good rotation would be established. 

 Unfortunately, in the West Indies, very little attention has 

 been directed to working out a proper system of rotation, as 

 has been done in Europe and North America ; and it is im- 

 possible, therefore, to lay down any precise rules for the 

 guidance of all planters. But every one can make experi- The import- 

 ments for himself, and it is not necessary, m the way of experiments, 

 experiment, to plant more than a small portion of land, say 

 50 or 100 feet square. A good planter will be always experi- 

 menting in one way or another, in order, practically, to find out 



