456 IRRIGATION FARMING. 



asmuch as an abundance of soil moisture and good 

 tilth are prime requisites for the development of avail- 

 able nitrogen, it is clear that a deficiency at any time 

 between July and September must reduce the yield by 

 lessening the rate of nitrate development, and the pre- 

 vention of this occurrence by supplementary irrigation 

 must tend to maintain a higher average. The case is 

 rendered still more critical by the fa(5l that when the 

 amount of soil moisture is drawn down very low, the 

 rate at which it can travel through the soil by capil- 

 larity to reach the roots is so much reduced by rela- 

 tively greater fricflion that growth must slow down 

 because the water cannot move rapidly enough to meet 

 the needs. 



It must be remembered also that the rains are never 

 as effedlive after the soil has been permitted to become 

 too dry, for the reason that it is retained so close to the 

 surface that it does not reach the roots, and a very large 

 percentage of it is returned at once to the atmosphere 

 through evaporation from the soil. In many states the 

 rainfall for June, July, and August may average more 

 than twelve inches, and yet a second crop of hay with- 

 out irrigation is a very rare possibility, chiefly because 

 the first crop so thoroughly dries the surface soil that 

 future rains are largely retained by it and returned to 

 the atmosphere without helping the crops or assisting 

 in developing plant-food. There may often be mois- 

 ture enough in the second and third foot so that a 

 small supplementary irrigation puts the soil in prime 

 condition, and hence a comparatively small cost when 

 compared with the demands of the arid region would 

 ensure a second and third crop of hay. Besides, it has 



