85 



removal of oxygen. II o direct evidence supports I.Iitscher- 

 lich's opinion that the carbon dioxide in the soil is 

 beneficial and that a degree of aeration sufficient 

 to remove this gas is detrimental. 



The conflicting nature of the evidence makes 

 impossible any final decision concerning the effect of the 

 carbon dioxide of the soil air on the roots or other por- 

 tions of plants. It is probable, however, that the nature 

 of plant and of soil affect the reaction and that a con- 

 centration of carbon dioxide toxic to one plant in a 

 given soil might not be toxic to other plants, or to the 

 same plant in another soil. Similarly in water-cultures 

 the plant grown and the composition of the solution may 

 affect the result. Whether the effect of carbon dioxide 

 on roots resembles its effect on shoots in that there 

 is a stimulation by small concentrations cannot be deter- 

 mined from existing data. The experimental cases of bene- 

 fit from application of carbon dioxide to the soil may 

 be explained by secondary reactions, such as effects on 

 micro-organisms 2 or the well-known effect of dissolved 



1. Bodenkunde, 2nd ed., p. 248 (1913 J. 



2% For instance, the stimulation of the 

 nitrifying bacteria by carbon dioxide. See Oweii,- 

 Guorgia Agr. Exp. Sta., Bull. _87 (1907). 



