DRAINAGE AND THE ROOT ZONE 245 



without removing the products from the root zone. (2) 

 Through direct chemical changes which result from good 

 aeration. (3) Through the activity of organisms which 

 not only form nitrates but produce carbonic acid and 

 other materials which increase the availability of the 

 mineral portion of the soil. The thoroughness of these 

 chemical changes is well illustrated by the uniform color 

 of a well-drained and well-aerated soil, in contrast to the 

 usually mottled color of poorly aerated and wet soil. 

 Drainage enables the plant-grower to make better use 

 of the food stored in his soil. 





r jr *' ^ . *= : ~ ^z: - . M -* v* u ^ - : : - r = *rv - '' ' 





Fig. 78. Cross-section of tile-drained soil, showing the elevation of the water- 

 table between lines of drains. 



117. Enlarges the root zone. Drainage deepens 

 and enlarges the root zone of the soil by the removal 

 of the gravitational water and by the admission of air. 

 Thereby the plant is brought into intimate relation 

 with a much larger volume of soil from which it may draw 

 moisture and food. It is thus enabled to withstand more 

 protracted periods of dry weather; it enjoys a more uni- 

 form climate, and has a larger food-supply, all of which 

 are conducive to a rapid growth and a larger yield. 



118. Reduces "heaving." Drainage reduces "heav- 

 ing," which results from freezing of a wet soil. When 

 water freezes, it expands one-eleventh of its volume. 

 In a saturated soil, this expansion can take place in 

 only one direction upward with the result that the 



