118 ELEMENTS OF AGRICULTURE 



We usually think of cultivation as meaning the pro- 

 cess of pulverizing the soil's surface by plowing, 

 spading, rolling, etc., and these are the ordinary 

 methods employed. But there are other methods of 

 cultivation besides mere surface cultivation, and two 

 that are oftentimes of great importance are drainage 

 and irrigation. They both affect the water supply of 

 soils, and water is probably the most important of all 

 plant foods. 



113. Drainage. — In Chapter XIX you were told how 

 soils lose their water supply, becoming dry and barren ; 

 on the other hand, there .are soils that contain too 

 much water. In many places the impervious stratum 

 lies near the surface of the soil, and the soil being shal- 

 low, becomes quickly filled with free water. If now 

 the impervious stratum has little slope the free water 

 drains away very slowly, and the only way the soil is 

 freed from it is through surface evaporation, which, 

 if the soil be covered with vegetation, is slow. The 

 drainage of the soil being poor and the surface evapo- 

 ration small, water accumulates and keeps the soil 

 constantly wet. It is in this way that swamps and 

 marshes are formed. The excessive amount of water 

 may be drained from such soils by digging ditches 

 through which the water may flow into some creek or 

 river. As open ditches interfere with surface cultiva- 

 tion, they are usually made into what are called under- 

 drains. In the bottom of the ditch a little culvert is 

 built of stone, brick or w T ood, the joints being left open. 

 The ditch is then filled in with earth, and the under- 

 drain is completed. The free water in the soil enters 



