THE PRINCIPLES OF TILLAGE 



107 



looseners (including weed cutters), and (3) soil firmers. 

 Tlie value of any oi! the three classes is determined by 

 (1) its efficiency in performing the work desired, (2) its 

 draft, (3) its cost, and (4) its durability. It is not our 

 purpose to discuss the draft or cost or durability of the 

 different types, although these are very important 

 phases of the subject, but to consider very briefly the 

 functions of the different classes of tillage implements 

 and their relative suitability for specific purposes. 



Plows. 



82. The Function of the Plow. — The function of the 

 plow is to cut off all the roots of plants, to turn the f ur- 



Stubble Bottom Z 



General Purpose 



Black Land 



Rod Breaker 



Fig. 37. — Types of Bottoms. 



row upside down and completely cover all vegetation and 

 litter, to pulverize the soil, to break up a hard subsoil, 

 to leave the surface as smooth as possible (in this 

 climate) after the operation and to do any or all of 

 these at the lowest possible cost. In plowing, plants are 

 killed by cutting off the roots below the crowns and by 



