290 Ground Water, Wells and Farm Drainage. 



foot, when both are well drained, and as much as 5 in the 

 second and third feet. 



360. Soil Better Ventilated by Drainage. The change of 

 air in wet soils after they have been well drained is very 

 much more thorough and this is perhaps the greatest bene- 

 fit due to drainage. 



There are several ways in which thorough drainage leads 

 to a more rapid exchange of air in the soil: 



1. Lowering the ground water enables both the roots of 

 plants, and animals like earthworms and ants, to penetrate 

 the soil more deeply, leaving passageways larger and freer 

 than existed before. 



2. When the deeper clays come to dry after being 

 drained shrinkage checks are formed in great numbers and 

 through these the air moves more freely. 



3. With the deeper penetration of soil air nitrates are 

 more freely formed, and with the larger amounts of soluble 

 salts the clay is flocculated, making a more granular text- 

 ure, which again admits the air more freely. 



4. When lines of tile are laid under a field 50 to 100 feet 

 apart they furnish an opportunity, with every change in 

 atmospheric pressure and of soil temperature, to force air 

 into and out of the soil, and so a line of tile laid in the soil 

 becomes a system for air circulation. 



5. With every heavy rain which causes percolation, 

 where the water can flow away, a volume of fresh air is 

 drawn into the soil after it, completely changing the air. 



361. Kinds of Drains. There are two types of drains: 

 (1) closed and beneath the surface after the manner of- un- 

 derground water channels; and (2) open, such as ditches, 

 which are in function like natural river channels. 



The closed forms are usually most effective, least in the 

 way, require less expense in maintenance and are most 

 durable and should generally be adopted, but there are cases 

 where surface ditches must be used. 



In the earlier history of underdraining closed drains were 



