ROTATION OF CROPS. 



The experience of practical farmers very early demon- 

 strated the necessity for adopting a system of changes in 

 the crops grown on the same soil. Thus, we find in the 

 writings of Columella, Varro, Theophrastus, and others who 

 in ancient times wrote on the subject of agriculture, distinct 

 rules laid down as to the course of cultivation to be pur- 

 sued in order to prevent the exhaustion of the soil, or, rather, 

 to prevent it from failing to produce a particular crop so 

 long as it was fertile for anything, and to enable it to make 

 full use of whatever manures were applied to it. 



In more modern times, the reasons why rotations are ne- 

 cessary have been, in a measure, explained by the aid of 

 chemistry, but we have not materially improved on the 

 practice of those who cultivated the soil 2000 years ago. 



The various crops appropriate different elements from the 

 soil, or the same elements in different proportions. Of 

 course, by raising the same crop year after year from the 

 same field, its quantity and quality not only yearly deterior- 

 ate, but the soil becomes exhausted of the special ingredi- 

 ents which go to support the growth of that particular 

 product, while it accumulates the elements especially adapted 

 to some other crop. 



