Drainage Ventilates the Soil 419 



place in the soil, which are essential to its fer- 

 tility, demand both free oxygen and carbon dioxide, 

 so that here is another need for good drainage, in 

 order that air may enter the ground in abundance. 



HOW DRAINAGE VENTILATES THE SOIL 



Where standing water would be found in holes 

 sunk 18 to 24 inches below the surface, capillarity 

 would hold the pores of a fine soil so nearly full 

 of water to the top of the ground that there would 

 be little room left for air to enter ; but when the 

 ground water is permanently lowered three or four 

 feet, as is done by underdraining, the roots of plants 

 penetrate the soil more deeply, and, as they die and 

 decay, leave passageways leading to the surface, into 

 and out of which the air readily moves. Earth- 

 worms, ants, and other burrowing animals penetrate 

 the ground more deeply, and open other ventilating 

 flues of much larger magnitude than those left by 

 the roots of plants, and so greatly increase soil ven- 

 tilation as a result of drainage. 



Then, again, when the deeper clays dry out, as 

 they will after underdrainage, shrinkage checks form 

 in them in great numbers, opening tiny fissures 

 through which the air moves more freely with every 

 change of temperature and pressure of the atmos- 

 phere above. With the deeper and more thorough 

 penetration of soil -air, carrying with it the car- 

 bonic acid developed near the surface, there begins, 

 through the agency of the soil water, a solution of 



