FARM DRAINAGE 



237 



lower level and then let gravity pull the water 

 through them to lower ground below. These pas- 

 sageways are called drains. 



Drains may be classed as : 



Surface drains which are shallow, open channels 

 made in the soil with a plow, hoe or other tool, to 

 carry off surface water. They are temporary and 

 need frequent renewing. 



Open-ditch drains are deeper, more permanent 

 water passageways around or across the fields. 



Surface and open-ditch drains take only surface 

 water. They also carry off surface soil and ma- 

 nures washed into them. They frequently become 

 choked or stopped by trash and soil, and are in the 

 way of cultivation and harvesting operations. 



Covered drains, under drains or blind ditches are 

 water passageways made of brush, poles, stones, 

 tiles, etc. (Figs. 80-81), placed in the bottoms of 

 ditches and then covered with soil. 



INFLUENCE OF COVERED OR UNDER DRAINS ON 

 FERTILITY 



Influence on soil water. 



Covered or under drains take not only surface 

 water, but also remove free water from the soil be- 

 neath down to nearly the level of the bottom of the 

 drains, and thus increase the area of film water. 

 Removing the free water enables the soil to absorb 

 more readily rain water falling on the surface and 

 therefore checks surface wash and the gullying of 

 fields. 



