96 DRY LAND FARMING 



to, the latitude being the same. Because of the influence 

 thus exerted by latitude and altitude on evaporation, it 

 has been claimed by high authority that 15 inches of an- 

 nual precipitation in Dakota or Montana will be as help- 

 ful in sustaining vegetation as 20 inches in southwestern 

 Nebraska and northwestern Kansas. 



The influence of humus in the soil is very potent 

 on the transpiration that will result, not only because it 

 lessens transpiration, but because it increases the mois- 

 ture supply available for transpiration. A soil well stored 

 with humus will sustain plant growth without languish- 

 ing in a time of drought for a much longer period than 

 a soil not thus prepared to resist the influences of drought. 

 But the best methods of storing the soil with humus in 

 dry areas have but imperfectly been worked out. The 

 crops that are best fitted to increase the humus supply 

 and the best methods of growing them are as yet but 

 imperfectly understood (see p. 420). 



The importance of subsoil moisture. The chief 

 function of water in the subsoil in dry areas is to fur- 

 nish a supply to the growing crops, when the supply 

 from the surface soil is insufficient to meet the needs of 

 the same. This is done by entering the root hairs that 

 penetrate between the subsoil particles, and by furnish- 

 ing additional water drawn from lower depths through 

 capillary movement. Winter wheat and winter rye are 

 frequently brought safely to maturity through water 

 from this source. Crops of spring grain may grow vig- 

 orously for a time and then fail because of the shortage 

 of water in the soil near the surface, whereas such fail- 

 ure would not have occurred had a sufficiency of moisture 

 been present in the subsoil. But the fact should never 

 be forgotten that the upward movement of subsoil mois- 

 ture will carry it into the air when not taken up by grow- 

 ing plants, or when such escape is not prevented bv the 

 presence of a dust mulch on the surface. 



