SOIL WATER 87 



maximum crops. In the northern part of the country, 

 this is particularly true during July and August. The 

 tillage of the soil is therefore of great importance, as a 

 means of absorbing and retaining as much water as pos- 

 sible for use during the months when the demand for water 

 is so great. 



When seeds are planted, it is very often desirable to 

 increase evaporation, so that the seeds that are near the 

 surface will be kept moist by the water as it rises to the 

 surface. This is accomplished by packing the ground over 

 the seeds by rolling the 

 field, or by packing it 

 over the row only, as is 

 done by a corn planter. 

 (See Figs. 44, 45, 46.) The 

 packing makes the pore 

 spaces smaller so that 

 the capillary movement of 



, ... , FIG. 46. A roller. Crushes clods and packs 



the Water Will be more the surface so as to keep the seed moist at 



* ne expense of increased evaporation. 



89. Dry-Land Farming. Two-fifths of the United States 

 is too dry to raise good crops without irrigation. (See Fig. 

 47.) Few crops can be grown successfully without twenty 

 or more inches of rainfall. In the past few years, consid- 

 erable attention has been given to a system of farming 

 that attempts to save all the rainfall of one or more years 

 for the use of a crop during the growing season. In this 

 way, the rainfall of two years can be used for the produc- 

 tion of one crop. Sometimes two crops are grown in three 

 years. In this system, the land is kept tilled during the 

 year when there is no crop, so that the rainfall may be 



