14 



LAND DRAINAGE 



5. Geneva Society Variable weather averages for one season, 

 Geneva (Schiibler) 



6. Temperature averages for one season, Mich. Exp. Station 

 (Bouyoucos) 



VENTILATION 



Soil ventilation does not differ essentially from house 

 ventilation. It involves the displacement of bad or viti- 

 ated air by pure air. There are several reasons why a 

 soil should be ventilated. 



20. Ventilation and food preparation. The breaking 

 down of the minerals of the soil and the consequent libera- 

 tion of contained plant-foods are favored by the presence 

 of the oxygen of the air. The processes referred to in 

 paragraphs 7 and 9 by which the nitrogen is changed, 

 step by step, from non-usable to usable forms, can take 

 place normally only when an abundant supply of oxygen 



