110 THE PRINCIPLES OF AGRICULTURE. 



these tracts, one continuous field may be formed out of 

 a number of plots, which is of great advantage in the 

 business of farming. 



6. It increases the warmth of the soil. 



The evaporation of water is a cooling process. It ab- 

 sorbs heat from the substance from which the moisture 

 escapes. An object may be cooled in hot weather by 

 covering it with a damp cloth. Damp clothing gives the 

 body a chill, not so much on account of the presence of 

 the moisture in the clothing, as from its evaporation in 

 drying. So wet land is always cool, because, on account 

 of the presence of water, evaporation is always going on. 



The warmth of soil is promoted by drainage, not only 

 by drawing the water away from beneath, and preventing 

 evaporation, but also by admitting warm air. 



Rotation of Crops. A knowledge of the fertility and 

 cultivation of the soil is not complete without an under- 

 standing of the principles involved in the rotation of 

 crops, or an arrangement of different crops to occupy the 

 same land successively. 



It is a familiar fact, that, when the same kind of crop is 

 raised upon land year after year, the yield becomes less 

 and less ; and that, where one crop fails to yield good re- 

 turns, another may flourish. This fact leads to the prac- 

 tice of changing frequently from one crop to another. 



The advantages of this practice are as follows : 



1. It makes use of all the elements of plant food. 



Different crops require the elements in different pro- 

 portions. It is not often that these elements are sup- 

 plied by the soil in the exact proportion required by any 

 particular crop. 



Potatoes, for example, require more potash than wheat, 

 and wheat requires more phosphoric acid than potatoes. 



