ROTATION OF CROPS. 



79 



piles should be covered with a thin layer of earth, to 

 be added to whenever bad-smelling gases are found 

 to escape. 



176. Professor Ville, a noted French writer on 

 fertilizers, calls a " complete manure" one that con- 

 tains nitrogen, potassium, phosphorus, and lime, in 

 suitable proportions to meet the demands of a given 

 crop. These four elements are the only ones likely 

 to be deficient ; and he advises farmers to purchase 

 the chemical salts containing these elements, and mix 

 them according to formulae given in his work on 

 "Chemical Manures." A good superphosphate with 

 nitre, or with some salt of ammonia, and one of po- 

 tassium, would constitute such a mixture. 



CHAPTER IX. 



ROTATION OF CROPS. 



177. THE fact has long been known that it is not 

 best to grow the same kind of crop on the same 

 land for a number of years in succession. Thus, 

 the yield of corn, wheat, oats, tobacco, or other crop, 

 if grown on the same land without change will grad- 

 ually diminish, while if these crops be made to alter- 

 nate, that is, first one and then another, the aggre- 

 gate product will be much greater. This change is 

 called rotation of crops. 



178. The advantages of rotation, or change of 



