THE MOISTURE IN THE SOIL 53 



3c. Capacity may be increased by under -drainage 



75. Drainage has an intimate relation to soil 

 moisture. By drainage is meant the means 

 employed for the removal of the surplus free 

 water. Surface or open ditches may serve as 

 conduits to carry off surface water, but as soil 

 drains they are failures. The correct method 

 for removing the surplus water of rainfall is to 

 cause it to sink into the soil and be removed 

 by under- drains. That which is removed by 

 surface flow fails to impart any beneficial effect 

 to the soil (69). 



76. Lands which are well under- drained are 

 porous. The rain which falls upon them passes 

 down quickly, and is not removed by surface 

 flow. It is removed only when the level of the 

 free water rises to the level of the drain. By 

 observing the action of drains which are of dif- 

 ferent depths, it has been found that after a 

 protracted drought the drains which begin to 

 flow first are those which are at the greatest 

 depth, showing that as the level of the free 

 water rises to the drain the flow begins, and 

 that it is not removed to any considerable ex- 

 tent in its downward passage. 



77. The sinking of the water through the soil 

 does more good than merely to supply moisture. 

 In the spring the rain is warmer than the soil, 



