62 AGRICULTURAL ENGINEERING 



the soil to warm up earlier in the spring, and to remain warm 

 later in the fall, thus indirectly increasing the length of the 

 growing season. The cooling effect of the evaporation of 

 water is known to all. 



The Soil Temperature Is Raised. In a manner similar 

 to that just explained, the soil is maintained at a warmer 

 temperature throughout the growing season, assisting in the 

 rapid growth of plants. 



Ventilation. Underdrainage causes the soil to be aerated ; 

 for as soon as the hydrostatic water is drawn away by the 

 drains, the space between the soil particles is filled with air. 

 This has a beneficial effect, since all plants require some air. 



Prevents Surface Wash. When the hydrostatic water 

 of the soil is drawn away by underdrainage, the soil is in a 

 condition to receive a very heavy rainfall before the water 

 will run off over the surf ace; or, in other words, underdrainage 

 will enable the soil to provide a large reservoir for rain water. 



Increases the Depth of Soil. As the soil becomes warmer 

 and aerated, the roots strike deeper, thus increasing the 

 depth of the soil available for plant food. 



Drouth. Strange as it may seem, well-drained soil 

 resists drouth better than wet. The greater fineness and 

 depth of the soil enable it to retain a larger amount of capil- 

 lary water, which is the water chiefly used by plants. 



The Action of Frost Is Reduced. Soil which is filled with 

 hydrostatic water expands upon freezing and is said to 

 "heave." Although the action of frost may be beneficial, 

 as previously explained, heaving is very injurious to certain 

 crops which are planted in the fall. If the ground water of 

 the soil is drained out, this action is almost entirely over- 

 come. 



