CHAPTER X 



DRAINAGE AND IRRIGATION 



Till taught by pain, men really know not what good water's worth. 



BYRON. 



THESE two special methods of soil improvement are closely 

 associated with each other in many places. In regions which are 

 irrigated throughout the entire growing season with an abun- 

 dance of water, it is often found advisable to remove surplus 

 water by underdrainage. This would not be necessary if the 

 soil and subsoil are open and very porous; but is found advis- 

 able if the subsoil is very heavy and compact. In regions where 

 the soil contains considerable black alkali with a superabundance 

 of sodium carbonate or similar salts, the effect of irrigation water 

 may be to dissolve the alkali salt which is deposited at the surface 

 during the process of evaporation. This is very injurious to plants. 

 If such fields be underdrained much of the surplus salt will be 

 washed into the drains and thus be removed from the soil entirely. 

 A combined drainage and irrigation system is the best plan by 

 which to improve such alkali soils. 



The student should understand that alkali as used in this 

 agricultural sense is not a true alkali in a chemical sense. It is 

 really a surplus salt which has an injurious effect, by "super- 

 saturing" the soil water, which causes plants to wilt because of 

 reversing the osmotic action in the roots. 



DRAINAGE 



It is often thought that drainage is needed only in places where 

 there is too much water. There are millions of acres of marsh 

 land requiring drainage for the removal of surplus water. But 

 there are also a number of other places which should be drained. 



Where Drainage Will Help. Soils which warm up too slowly 

 in the spring may be improved by underdrainage. There are often 

 limited areas where water stands near the surface, but is not 

 exposed to view; these are improved by lowering the water table. 

 Drainage is desirable in places where the water is abundant in 

 the early part of the season, bat where plants actually suffer from 

 drought later in the summer, The effect of drainage in such 

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