CONDITIONS AFFECTING SOIL FERTILITY 49 



water after a rain. Ponds and marshes are usually at the 

 level of the water table. 



Drainage. In the eastern United States it is estimated that 

 there are a hundred million acres of swamp that could be 

 rendered useful by drainage. All soils have to be drained 

 before cultivated crops can be grown in them. The majority 

 are drained naturally. Sand is often too well drained. When 

 the saturated con- 

 dition of the soil 

 results from the 

 seepage of water 

 from higher levels, 

 tile drains may be 

 used to carry it 

 away, but when 

 the land lies very 

 little above the nat- 

 ural drainage, tile 

 drains would be 

 useless and open 

 ditches must take 

 their place. Lands 

 that are not per- 

 manently wet are 

 often benefited by 

 tile draining. The 



removal of the surplus water deepens the area into which 

 the roots of plants can spread, warms the soil by admitting 

 the air, and promotes further weathering of the subsoil. 

 Draining wet lands may actually give the plants more water 

 by increasing the area from which the roots can absorb, 

 thus making them more drought-resistant. Drainage also 

 makes it possible to begin the work on wet lands earlier in 

 the spring. 



Photograph by II . L. Ilollister Land Co. 



FIG. 23. A check gate in an irrigation ditch 



This holds hack the water and causes it to flow 

 through smaller channels into the fields 



