LAND DRAINAGE 631 



spaces. This results in better ventilation. The free 

 movement downward through the soil of the waves of 

 saturation accelerates the process of deep soil ventilation 

 by driving the contaminated air out through the under- 

 drains while fresh air is drawn in behind the wave of soil 

 moisture. 



3. The removal of the excess moisture by drainage 

 permits the soil to maintain a higher average tempera- 

 ture. The high specific heat of water as compared with 

 the soil causes the presence of water to be the chief deter- 

 mining factor in soil temperature. Further, the process 

 of evaporation of the excess water from the soil requires 

 a tremendous amount of heat. The use of solar heat to 

 warm useless water and to remove it by evaporation is 

 avoided by draining away this excess. Drained soil not 

 only maintams a higher average temperature in summer, 

 but warms up earlier in spring to a temperature for 

 planting seeds. This gives a longer growing season. 



4. The improved ventilation resulting from drainage 

 permits the roots of plants to penetrate deeper into the 

 soil, where they come in contact with a larger supply of 

 moisture and food. One of the indications of the need of 

 drainage is the shallow root development of crops. Stag- 

 nant water in a saturated soil is as resistant to the pene- 

 tration of upland crops as is the hardest rock (see Fig. 

 63). 



5. The improved physical condition of the soil that 

 results from drainage permits the retention of a larger 

 amount of film water, and this, in time of drought, re- 

 sults in a much larger available supply of moisture to 

 the crops. 



G. The improved physical condition of the soil permits 

 better internal circulation of water, by which the films are 



