34 ENGINEERING FOR LAND DRAINAGE. 



The benefit as explained by some is due to the dry- 

 ness of the covering, which is secured by frequent cul- 

 tivation, and which induces the formation of a new 

 moisture line just beneath the surface. Another ex- 

 planation, and the more tenable one, is this: The rain- 

 fall saturates the surface and to some extent causes the 

 soil particles to assume a new and closer relation to 

 each other. These when dried by the sun and wind 

 form a thin surface crust, which increases the tension 

 and capillarity of the soil particles so that moisture 

 from below is carried rapidly to the surface and evapo- 

 rated. Cultivation breaks up this compact condition 

 of the soil and retards the capillary action of its surface 

 layer. Frequent surface culture, w^hich is now recog- 

 nized as a most important adjunct to successful soil cul- 

 tivation, is made possible by underdrainage, since the 

 soil can be cultivated soon after any rain without risk 

 of injury. 



