MANAGEMENT OF THE SOIL 



19 



controls the depth and position of the plow by means 

 of levers. 



The disk plow is entirely new in principle. It 

 consists of a large circular knife or disk of thin steel 

 which is pressed into a concave shape and mounted 

 so as to revolve on a central axle. It is set so that 

 the sharp edge enters the soil almost vertically and 

 the furrow slice is lifted and turned over by the 

 concavity of the revolving disk. Disk plows are 



Double Disk Plow. 



always riding plows. They may be single or arranged 

 in gangs. They are very useful for certain pur- 

 poses, especially in handling sticky refractory soils 

 in which ordinary turning plows do not scour 

 well. 



Formerly plows were nearly always drawn by oxen. 

 In tropical countries this is still the prevailing cus- 

 tom. The first cost of oxen is less than that of 

 horses or mules ; with good pasturage they can work 

 on that alone without requiring expensive grain 



