96 SOILS AND FERTILIZERS 



but is so treated as to permit of a very small amount of 

 evaporation. The surface is constantly kept loose, 

 being tilled after each rain of any considerable amount. 

 A dirt mulch is thus formed which reduces evaporation 

 to a minimum, and by the use of a subsurface packer 

 capillary action is stimulated so that moisture is brought 

 up from the subsoil to the seeds or roots of the plants. 

 After the crop is planted, the soil is kept loose at the 

 surface until the crop is matured. The moisture con- 

 served through one year by this system is often sufficient 

 for the maturing of crops through the next year. A 

 farmer in the arid region having a 32O-acre farm would, 

 then, have 160 acres in crop each year and 160 acres 

 lying fallow, that is, unused by a crop, but tilled during 

 the summer. The system is often called the Campbell 

 system, after its originator. 



The principles of dry farming might be put to 

 good use in both arid regions and those where there 

 is a good winter rainfall. In the former areas one 

 application of water would be enough for the entire 

 season. In areas having a winter rainfall the moisture 

 might be so conserved that irrigation would be un- 

 necessary. 



Amount of Water needed in Irrigation. Some au- 

 thorities claim that fifty thousand gallons an acre are 

 necessary to mature a farm crop by irrigation, but so 

 much depends upon the character of the soil and the 

 subsoil, and also upon the climate and the crop to be 

 raised, that it is difficult to give any accurate estimates. 

 If the climate is humid, less water will be required than 

 in a dry climate. It is better to apply a moderate 

 amount at intervals of several days than to put too much 

 on at a time. 



