80 DRY LAND FARMING 



While the degree of evaporation is probably less 

 important than the amount of the precipitation, it is 

 nevertheless greatly important. In areas far southward 

 in the dry belt, the precipitation called for to produce 

 plant growth is very considerably more than what is 

 called for to effect the same in areas of the same alti- 

 tude but located far to the northward (see p. 95). 



The methods by which water may be retained in 

 the soil until it is needed involve consideration of the 

 handling of the soil in all its phases, including: (1) the 

 breaking up of the same; (2) the subsequent plowing; 

 (3) the various processes of tillage, including packing, 

 discing, harrowing and rolling; (4) subsoiling, and (5) 

 succession in the crops that are to be grown. 



The plants that may be grown with the greatest 

 success is in itself an important study. The species of 

 plants not only differ very much in their adaptation to 

 dry areas, but this is also true of varieties of the same 

 species. To attempt to grow those lacking in adaptation 

 would not be wise. The value of correct information 

 along these lines cannot easily be overestimated (see 

 chapter X). 



Water in semi-arid soils. Water occurs in all soils : 

 (1) as free water; (2) capillary water; (3) hygroscopic 

 water, and (4) the water that runs away and is lost to 

 the soil. ,In semi-arid areas the free water and the water 

 that runs away is much less abundant than in humid 

 areas. The distinctions thus given are not sharply 

 drawn, as will be apparent from what is said below. 



Free water, sometimes called gravitational water, 

 is that which fills the pore spaces between the soil grains 

 and moves down through the soil by gravity. When 

 present .in excess it excludes the air so as to hinder 

 healthy plant growth and in many instances to confine it 

 to growth that is not of much value. Passing down into 

 the subsoil, it may reach a point where further descent 



