252 ENGINEERING ON THE FARM 



percentage of the rainfall is conserved in the soil until there 

 is a sufficient quantity stored up to produce a crop. A crop 

 is raised on an average of every other year. As soon as a 

 crop is harvested the soil is plowed and packed by means of 

 special tools, and a sufficiently loose covering is kept over 

 the surface to absorb moisture and prevent evaporation. 

 It is cultivated just enough to keep down vegetable growth 

 and maintain proper soil conditions. Plants which use a 

 minimum of water in their growth and fruitage are selected. 

 Dry farming cannot increase the quantity of moisture in a 

 region. It is only a method of conserving as much as pos- 

 sible of the water that naturally reaches the soil. The time 

 that must elapse between crops must be such that sufficient 

 water is furnished by the natural rainfall minus the small 

 loss that will inevitably occur. 



Fall irrigation. The storage of moisture in the soil is 

 used in other ways than that of dry farming. After harvest 

 a large quantity of water was formerly permitted to go to 

 waste because there was no immediate need for it in crop 

 irrigation. Water which is available at periods of the year 

 when there is no plant growth is now frequently allowed to 

 saturate tracts of land not included in the farming areas of 

 the more valuable crops. The water thus stored in the soil 

 is sufficient the following year to produce grass and crops 

 that mature early or do not require a large quantity of water 

 in their growth. This method of irrigation is known as fall 

 irrigation. 



Winter and spring irrigation. When the water is per- 

 mitted to saturate the ground during the winter months 

 under practically the same conditions as described for fall 

 irrigation, it is known as winter irrigation and can be prac- 

 ticed only where the winters are mild and open. Early 

 spring irrigation consists in saturating the soil with water 

 that has been collected in reservoirs from the melting of 

 snows or from winter rains. This water is applied before 

 the growing period begins. 



