DRAINAGE 159 



of the crops ordinarily penetrate the soil. Good tile 

 drainage lowers this water level. Stiff clay soils both 

 on low, flat land and upon hillsides often hold an ex- 

 cess of surface water, and need drainage. 



Aerates the soil. As was stated above, plants require 

 air at the roots as well as about their foliage. The 

 presence of air in the soil promotes the action of bac- 

 teria, the germs that aid in breaking up the humus, as 

 was shown in a previous lesson, and in the fixation of 

 nitrogen in legumes. Air circulating through the tiles 

 tends to draw the air through the soil from the surface 

 above and thus to aerate the whole root-feeding area 

 of the plants. 



Warms and sweetens the soil. In wet soils so much 

 heat is used in evaporating the water that these soils 

 never become warm. Often such soils are sour, and 

 cannot become sweet until the water is drained off and 

 the heat and air let in. Well drained land warms up 

 earlier in the spring, facilitating earlier planting, thus 

 extending the growing season of the crop. 



Conserves moisture. We were told a moment ago 

 that drainage removes the excess of moisture, and it 

 may seem a contradiction to state that drainage con- 

 serves the moisture, but this fact can be shown. Soils 

 that are aerated and tilled, as good drainage permits, 

 are more porous and are in better condition physically 

 both to receive and hold the rainfall and to promote 

 capillarity from the table water below. 



Methods of drainage. The tile system of drainage is 

 better than the open ditch, though more expensive. The 



