SOIL CONDITIONS AFFECTING PLANT GROWTH 51 



where tends to take too much water, with loss not only to 

 others on the same irrigation system, but also to himself. 



Air Supply. 



It is well known among farmers and gardeners that soil 

 aeration is essential to fertility but exact measurements are 

 difficult to obtain. The phenomena are more complex than 

 appears at first sight, involving two wholly distinct factors : 



1. The necessity of a supply of oxygen to the plant root. 



2. The toxic effect of the carbon dioxide which invariably 

 accumulates in a non-aerated soil or other medium. 



Moreover, plants vary considerably in their sensitiveness to 

 these factors. 



The simplest case is seen in water cultures where aeration 

 produces marked effects. In the experiments of Hall, 

 Brenchley, and Underwood (izic) the amounts of dry matter 

 produced were as follows : 



The more recent experiments of E. E. Free (looa) show 

 that plants do not all stand in equal need of oxygen. Buck- 

 wheat was grown in water cultures through one set of which 

 was blown air, and through others nitrogen, oxygen, and 

 carbon dioxide respectively. No difference was observable 

 between the plants supplied with nitrogen and those supplied 

 with air or oxygen : they all grew normally to maturity. In 

 this case, therefore, the root apparently can dispense with 

 gaseous oxygen. When, however, carbon dioxide was given 

 the plants sickened and wilted within a few hours, and died 

 in a few days. 



Stiles and Jorgensen (272) have confirmed this difference 

 between barley and buckwheat. 



Soil experiments are more difficult to carry out. B. E. 



