272 ROCKS AND SOIL 



called a plowshare, to a certain depth into the soil, and then turning the 

 slice of soil over upon itself in the furrow. The slice is usually laid at 

 an angle of 25 to 50 (half a right angle is 45) with a horizontal plane, 

 so that the edge of the slice projects from the ground. Plowing not 

 only loosens the soil, and breaks it up, but it also brings to the surface 

 new material for the plant's growth. Modern agriculture has greatly 

 increased the depth of plowing, as well as its extent. Primitive peoples 



Fig. 230. 

 A Disc Plow. The whirling discs are on the right. Courtesy of Deere and Co. 



merely scratched the surface, and were able to till only a limited 

 area of land. 



Cultivating is shallow plowing, or stirring, of a soil, usually after a 

 crop has begun to grow. Its purpose is chiefly to keep the surface soil 

 in a powdered condition, so that it will retain moisture. Cultivating 

 also destroys weeds. In dry fanning frequent cultivating is carried 

 on; this produces a deep dust mulch, or covering, which prevents the 

 evaporation of water from the surface, and yet permits moisture to 

 rise from the subsoil by capillary action. 



Plows and cultivators are of many types; each is good for a particu- 



