IRRIGATION 267 



might gradually become so alkaline as to be worthless 

 may for a long time be kept suitable for the growth of 

 crops. By using large quantities of water with natural 

 or artificial underdrainage the excess of alkali may be 

 slowly washed out of some soils. In some areas the 

 entire engineering plan for irrigation needs to be ar- 

 ranged with drainage systems so as best to avoid the 

 accumulative injuries of alkali deposited by irrigation or 

 seepage waters. 



Crops needing irrigation All farm, garden and horti- 

 cultural crops may profitably be irrigated, where water 



Figure 169. Turning water from field side ditch into furrows among garden crops. 



is inexpensive, at least in dry seasons. Under rare con- 

 ditions, forest crops may be irrigated profitably. Where 

 water is expensive and the rainfall is sufficient during 

 most years, irrigation can be afforded only for such 

 expensive crops as small fruits and vegetables. 



The time of the year in which to apply water to the 

 various crops, is a matter of detail which can be decided 

 only with a knowledge of the local conditions of any 

 crops and of the methods of farm management of any 

 e^iven area. It is often necessary to apply water at or 

 before planting time so that the seeds will germinate 



