200 THE OLIVER PLOW BOOK 



that of a knife blade when one whittles a stick. The 

 disk has the advantage over the knife blade in that the 

 edge of the disk cutting the soil is constantly changing. 

 For this reason the edge of a disk will retain its sharp- 

 ness a great deal longer than the edge of a share. This 

 fact very probably has a great deal to do with the 

 superiority of this type of plow over the mouldboard 

 plow in exceedingly hard ground. 



The principle of the draft of a disk plow is entirely 

 different from that of a mouldboard plow. The shape 

 of the share and mouldboard of the mouldboard plow 

 cause the bottom to be pulled into the ground. On the 

 other hand a disk plow must be forced into the ground 

 by weight and draft combined. 



In operating a disk plow the hitch in front should 

 always be adjusted so as to keep the front of the plow in 

 the ground and sufficient weight added to the rear to 

 keep it in the ground at the desired depth. 



