CHAPTER IX. 

 EELATION OF AIR TO SOIL. 



NEEDS OF SOIL VENTILATION. 



Air in the soil in which crops are to be grown is as es- 

 sential to the life of the plants as the air in a stable is 

 to the life of the animals housed. 



Careful observations and lines of experimentation have 

 proved, in many ways, that when oxygen is completely ex- 

 cluded from seeds that are otherwise under good conditions 

 for germination they fail to start. It has been found, too, 

 that even after a seed has begun to grow, if the oxygen 

 supply is cut off, it makes no farther progress. Growth 

 does take place in seeds in a very dilute atmosphere of oxy- 

 gen, but after the amount has been reduced below iV of 

 the average in the air the plants advance very slowly and 

 are sickly. 



A soil in the best condition for crops must permit of 

 ready entrance of fresh air and an abundant escape of 

 the air once used ; in other words, like the stable, it must 

 be well ventilated. This ventilation is needed : 



(1) To supply free oxygen to be consumed in the soil. 



(2) To supply free nitrogen for the use of the free- 

 nitrogen-fixing germs. 



(3) To remove the excess of carbon-dioxide which is 

 set free in the soil. 



238. Needs For Free Oxygen in the Soil. Free oxygen in 

 the soil is required not only by the seeds, when they are 

 germinating, but throughout the active life of the plant 

 in order to permit the roots to live, for they, too, must 

 breathe. 



Then in the conversion of the nitrogen of humus, manure, 



