TILLAGE AND CULTIVATION 



65 



you like, because there is no "raw" subsoil except in 

 low, moist ground. And this deep plowing should be 

 done, as a rule, so as to enable the roots to go as deep as 

 possible for moisture and plant food. But we should 

 not plow to just the same depth every year, because 

 then we may form a hard layer just below the plow 



FIG. 36. Various kinds of plows: / is a gang plow, turning three furrows 



at one time. 



that will prevent the roots from going down. Such a 

 hard layer formed by plowing is called a "plowsole" 

 (see figures 28 and 35, pi.}. 



Instead of the turning plow we now frequently use the 

 disk plow, which consists of two or more revolving sharp- 

 edged steel disks set on an axis, which itself may be set 

 either straight, or slanting so as to move the soil partly 

 sideways and loosen it very thoroughly (see figure 37). 



