22 TILE DRAINAGE 



that it is only from the air which finds its way into the soil 

 that the bacteria, which live in leguminous plants, can 

 get their nitrogen, we see again the importance of thor- 

 ough ventilation. Drainage helps to ventilate a soil in 

 several different ways: (1) The soil drys out deeper and 

 in so doing shrinkage cracks are formed, through which 

 the air will readily pass; (2) the lines of tile themselves 

 form passageways or chimneys through which the air is 

 forced with every change in atmospheric pressure; (3) 

 after a heavy rain the air in the soil is forced out by the 

 water and, as this passes off through the drains, a fresh 

 supply of air is drawn in behind it. 



A third effect of drainage is to increase the room which 

 the roots may occupy. Most of our cultivated plants will 

 naturally send their roots down three to four feet in the 

 soil. If the gravitational water has not been removed to 

 this depth, the roots will be forced to make a shallow 

 growth and will be unable to utilize the full resources of 

 the soil. Drainage will so lower the ground water that 

 roots may penetrate to their normal depth. 



Another very important effect of drainage is that the 

 soil is made warmer. We know that evaporation is a cool- 

 ing process, as is illustrated by the wet and dry bulb ther- 

 mometer. When the surplus water in the soil during the 

 early spring can be removed downwards through the drains 

 instead of upwards through evaporation at the surface, the 

 temperature of the soil is maintained considerably higher. 

 Besides this, the water which falls as rain during the early 

 summer is always warmer than the soil and if it can be 

 absorbed and allowed to percolate down to the drains the 

 soil will be somewhat warmed. Experiments have shown 

 that the surface foot of a soil well drained is five or six de- 

 grees warmer than the same soil undrained. This differ- 

 ence in temperature will very materially hasten the ger- 

 mination of seeds and the length of the growing season 

 will thus be increased. Not only in this way is the grow- 

 ing season lengthened but a drained soil can always be 



