90 SOILS AND FERTILIZERS 



having a course with uniform fall to the outlet, it passes on till dis- 

 charged. The upper end of the tiling system is not open. The out- 

 let is the end that must be kept open and free from obstruction. 



IRRIGATION 



In the United States, as well as in other countries, are 

 vast tracts of land where rain seldom falls. There are 

 other large tracts where rain falls during certain portions 

 of the year, but not in sufficient quantities to make them 

 productive. (In fact in very few portions of our coun- 

 try is there moisture enough in every season to insure 

 the maximum crops.) From the Atlantic seaboard to 

 the Rocky Mountains the amount of water precipitated 

 annually gradually decreases, the extreme western por- 

 tion of this area being arid or semi-arid in character, 

 because of lack of moisture. The last half century has 

 seen the unproductiveness of nature supplemented by 

 man's work, in that a way has been found to supply the 

 needed water to these arid wastes. The process by 

 which water is supplied, other than by natural means, is 

 called irrigation. 



Sources of Water Supply. Water for this purpose 

 is often obtained from wells, ponds, lakes, and springs. 

 Sewage from cities is sometimes used on farms lying near, 

 but most of the water used comes from streams. When 

 there is any likelihood that the stream from which water 

 is taken will dry up during the season when water is 

 needed, reservoirs are built so that a greater part of the 

 winter rainfall may be stored. 



Uses of Irrigation. - - The water applied by irrigation 

 may be useful to the plant in two ways : First, it may 

 be of direct benefit in dissolving and carrying into the 

 plant the various foods found in the soil particles. 



