206 Physics of the Soil. 



fresh supply from above entering, an abundance of air 

 is carried with it for the needs of the roots. 



241. Excessive Soil Ventilation. The higher temperature 

 of a pile of open horse manure, as compared with that of 

 the closer heap of cow-dung, illustrates how important the 

 free and rapid access of air to tbs interior is to the forma- 

 tion of the ammonia, for the difference in temperature in 

 the two cases is largely due to a difference in the rate of 

 fermentation, and this to the too rapid entrance of air. 

 In these cases the air is entering too rapidly and a loss 

 of nitrogen is the result. And the same thing may occur 

 in a too open soil. Indeed, the small amount of humus in 

 the sandy soils is in a large measure due to the freer ac- 

 cess of air to the interior. 



It is for this reason that unusual care must be exercised 

 to keep the supply of humus in these soils up, not only 

 because of its need for plant food, but because it enables 

 the sandy soils to hold more water, and this in turn makes 

 them less readily penetrated by the air and the humus does 

 not waste as rapidly. 



242. Return of Carbon-Dioxide to the Air. It is of course 

 necessary to the continuance of plant life that the vast 

 systems of roots which are developed in the soil should be 

 broken down, first into humus and then into carbon-dioxide, 

 water and free nitrogen, and all of the processes concerned 

 in these changes demand free oxygen taken from the air 

 and the escape of the carbon-dioxide and nitrogen gas set 

 free, and here again is ample soil ventilation necessary. 



243. The Fixing of Free Nitrogen In the processes of 

 symbiosis discussed in (101), which lead to the removal of 

 the free nitrogen of the air in the soil and soil moisture. 

 and the conversion of it into organic compounds suitable 

 for the food of higher plants, soil ventilation is necessary 

 in order to supply both the oxygen and nitrogen of the air 

 which the micro-organisms are obliged to use in carrying on 

 their life processes. 



