218 HOW CROPS FEED. 



little to !id(l here to wliat lius been rcinarkt'd in previous 

 paragrajths. 



Free Oxygen, as De Saussure and Traube have shown, 

 is indispensable to growth, and must tlierefore be access- 

 ible to the roots of plants. 



The soil, being eminently i)orous, condenses oxygen, 

 Blumtritt and KeicJiardt indeed found no considei-able 

 amount of condensed oxygen in most of the soils and sub- 

 stances they examined (p. 167) ; but the experiments of 

 Stenhouse (p. 169) and the well-known deodorizing effects 

 of the soil upon fecal matters, leave no doubt as to the 

 fact. The condensed oxygen must usually spend itself in 

 chemical action. Its proportion would appear not to be 

 large ; but, being replaced as rapidly as it enters into com- 

 bination, the total quantity absorbed may be considera- 

 ble. Organic matters and lower oxides are thereby ox- 

 idized. Carbon is converted into carbonic acid, hydrogen 

 into water, protoxide of iron into peroxide. The upper 

 portions of the soil are constantly suffering change by the 

 action of free oxygen, so long as any oxidable matters 

 exist in them. These oxidations act to solve the soil and 

 \ender its elements available to vegetation. (See p. 131.) 



Free Nitrogen in the air of tiie soil is doubtless indiffer- 

 ent to vegetation. The question of its conversion into 

 nitric acid or ammonia will be noticed presently. (See p. 

 259.) 



Carbonic Acid. — The air of the soil is usually richer in 

 carbonic acid, and poorer in oxygen, than the normal at- 

 mosphere, while the proportion (by volume) of nitrogen 

 is the same or very nearly so. The proportions of car- 

 bonic acid by weight in the air included in a variety of 

 Boils have alrea'ly been stated. Here follow the totaJ 

 quantities of this gas and of air, as well as tlie composi- 

 tion of the latter in 100 parts by volume, as determined by 



