DRAINAGE 59 



2. Land kept continually wet by water appearing at the 

 surface, having seeped or passed underneath the surface 

 from land at a higher level. Such a condition is due to the 

 action of springs. 



3. Flat land underlaid with an impervious stratum of 

 clay which prevents the water from sinking downward 

 through the soil. Often this condition is represented by an 

 old lake bottom. 



4. Lands on which certain crops are grown, such as rice 

 fields or meadow lands, to which irrigation water may be 

 applied and removed at will. 



5. Lands subject to overflow by rivers or tides. 



Kinds of Soils. The kind of soil to be drained must by 

 all means be considered in connection with the planning of 

 farm drainage. The amount of capillary water that the 

 soil will hold varies largely with the fineness of the particles; 

 but a very fine soil will not allow water to pass through it 

 quickly, and for that reason is designated as a retentive soil. 

 There are other factors involved besides the fineness of the 

 soil particles; for example, the working or 'mixing of a finely 

 divided soil, such as clay soil, while filled with water tends to 

 make it impervious, or water-tight. 



An open soil is one through which the water will pass 

 quickly, and in which the pore space is not so finely divided 

 as in a retentive soil. The volume of the space between the 

 soil particles may be greater in the retentive soil than in the 

 open soil, as this space generally increases with the fineness of 

 the particles. 



Kinds of Underdrainage. All soils need underdrainage, 

 that is, the hydrostatic or ground water should be drawn 

 off from the soil in some way. In most cases this under- 

 drainage is provided by nature, and the ground is said to have 

 natural underdrainage. The same may be true where the 



