HARROWS FOR DRY-FARMING 311 
thinned effectively. At other timesit may be observed 
in the spring that the rains and winds have led to 
the formation of a crust over the-soil, which must be 
broken to let the plants have full freedom of growth 
and development. This is accomplished by slanting 
Cnty: 
Fie. 80. Spring tooth harrow. 
the harrow teeth backward, and the crust may then 
be broken without serious injury to the plants. The 
smoothing harrow is a very useful implement on the 
dry-farm. For following the plow, however, a more 
useful implement is the disk harrow, which is a com- 
paratively recent invention. It consists of a series of 
disks which may be set at various angles with the line 
of tractionand thus be made to turn over the soil while 
at the same time pulverizing it. (See Fig. 81.) The 
best dry-farm practice is to plow in the fall and let the 
soil lie in the rough during the winter months. In the 
