THE FORMS OF SOIL- WATER 179 



sistance offered to water movement. The depths for laying tile 

 in a heavy soil range from one and a half to two and a half 

 feet, while in a sand the tile may often be placed as deep as 

 four feet below the surface. It is evident also that the less 

 deep a tile drain is laid the less distance on either side it will 

 be effective in removing the water; consequently on a clay 

 soil the laterals must be relatively close as compared to the 

 interval generally recommended for a sandy soil. A rational 

 understanding of the movements of gravitation water is 

 clearly necessary in the installation of tile drains not only 

 that the system may be efficient, but also that a minimum 

 effective cost may be realized. 1 



The water lost from the soil by drainage is of especial in- 

 terest in plant production because of the large amounts of 

 nutrient elements carried away each year. Such loss is par- 

 ticularly important in regard to the lime and nitrogen. 2 The 

 equivalent of approximately 500 pounds of sodium nitrate 

 and 1000 pounds of calcium carbonate have been known to 

 leach from an acre of bare soil every year under humid con- 

 ditions. 



classes of soils. As usual, all of the moisture data is expressed as per- 

 centage based on absolutely dry soil. 



Sandy Clayey 



Soil Soil 



Specific gravity 2.67 2.65 



Volume weight 1.60 1.20 



Pore space 40.0% 54.8% 



Hygro. coefficient 1.0% 10.0% 



Optimum moisture (average) 10.0% 30.0% 



Maximum field capacity 17.0% 44.0% 



Air space at hygro. coefficient 38.4% 42.8% 



Air space at opt. moisture 24.0% 18.7% 



Air space at max. field capacity 12.8% 1.9% 



Possible free water 8.0% 1.6% 



See Kopecky, J., Die physikaliscJien Eigenschaften des Boden; 

 Internat. Mitt f. Bodenkunde, Bd. IV, Heft 2-3, Seite 138-180. 1914. 



1 For a more complete discussion of tile drains, see Chap. X, para- 

 graph 110. 



a Lyon, T. L., and Bizzell, J. A., Lysimeter Experiments; Cornell 

 Univ. Agr. Exp. Sta., Memoir 12, June, 1918. 



