SOIL WATER 89 



4. By the following formula, calculate the air space present. 



% air space = % pore space - (% water X ap. sp. gr.) 'Ans. 24.7 %. 



5. The wilting point in this soil is 4 percent. What is the per- 

 centage of available water ? Weight of available water per cubic 

 foot ? Per acre foot ? Arts. 6 %, 6 lbs. and 261,360 lbs. 



Exercise VIII. — Tile drainage. 



If possible, have the class install a short drainage system. They 

 should dig at least part of the ditch, grade the bottom, lay the tile 

 and build the outlet. The explanation of every point involved as 

 the work proceeds will give such an exercise great practical value. 

 It will also make the classroom work much more effective. 



If drainage operations are being conducted in the near vicinity, 

 the class should by all means bp taken to inspect them. The 

 general plan of the work, as well aJs the more detailed phases, should 

 be explained by the teacher. Materials and illustrations may also 

 be obtained for later discussion and study in the classroom. If 

 ditching machinery is being utilized, it also should be given consider- 

 able study. 



Early in the spring, while the soil is still wet, a field trip might 

 well be taken. The need of drainage, the movement of water 

 through soil, the effectiveness of drainage, the entraiice of water into 

 a drainage system, the movement of water through tile, good and 

 poor outlets and the drainage of roads could be studied with profit. 



