SURFACE WATER, DRAINAGE, AND IRRIGATION 311 



which are carried in solution into the soil by the surface 

 water. 



Where the subsoil is porous, natural drainage is usually 

 sufficient to remove the excess of water. Where the sub- 

 soil is a heavy clay, 

 artificial drainage 

 must be employed. 

 Most farms can be 

 improved by tile 

 drains properly lo- 

 cated, and many 

 farms need a com- 

 plete system of 

 drains laid from 

 two to six rods 

 apart. (Figure 

 267.) 



Tile drains, in 

 addition to remov- 

 ing excess soil 

 water, actually 

 serve .to protect 

 plants in times 

 of dry weather. 

 When the excess 

 soil water is quickly 

 removed, the plant 

 roots grow much 

 deeper than in soil that is wet and cold. These plants 

 with deep roots are then able to withstand considerable 

 dry weather, since the roots are able to use the capillary 

 water of the subsoil. 



Irrigation. About two fifths of the total area of the 



FIG. 267. Tiling Level Lands, Illinois. 



