88 GENERAL CONSlDEKATiONS 



proper rotation of crops. It is, with few exceptions, 

 extremely unwise to devote any given piece of land 

 continuously to the same crop. Each crop draws 

 on the food elements in the soil in different propor- 

 tions, and where only one crop is planted the ten- 

 dency is to exhaust the soil unequally. If this were 

 the only difficulty, it would be easily overcome by 

 the proper selection of fertilizers, but there are many 

 others. Some crops, like cotton, require absolutely 

 clean cultivation throughout the season. This, as 

 has been previously pointed out, tends to the rapid 

 exhaustion of vegetable matter. Other broadcast 

 crops like the small grains cannot be cultivated at 

 all, and if planted continuously the land becomes 

 very foul with noxious weeds. Some crops occupy 

 the land so continuously that they do not give time 

 for the growing of leguminous catch crops for green 

 manuring. The most imperative reason for crop 

 rotation is, however, its usefulness in checking the 

 ravages of diseases and insects. When the land is 

 planted continuously to any given crop for a term of 

 years, it makes ideally favorable conditions for the 

 rapid multiplication of the diseases and insects pe- 

 culiar to that crop. There are many instances in 

 which, owing to continuous cropping, the land has 

 become so infested with disease that the cultivation 

 of the crop has had to be entirely abandoned. The 

 bacterial blight of tomatoes and potatoes and the 

 Fusarium wilt of watermelons and cotton may be 

 mentioned as instances of Southern diseases that 

 often become very destructive when these crops are 

 grown continuously. Some investigators have re- 



