106 AERATION AND AIR-CONTENT. 



In a final paper on indigo wilt (1920), they state that the conclusion 

 is irresistible that the trouble results from the destruction of roots 

 and nodules under conditions in which regeneration is impossible. 

 If floods cause the ground-water to rise, or if heavy rainfall water- 

 logs the surface soil for long periods, the defective aeration makes 

 root regeneration very difficult, and wilt ensues. In confirmation, 

 it has been shown that other deep-rooted species exhibit wilt, while 

 varieties of these with shallow roots do not. Moreover, wilt is 

 common in years of heavy rainfall, and rare or of slight importance in 

 dry years, while it is seldom found in porous soils. 



Clements (1920 : 85) recognizes that plants may serve as indicators 

 of good or bad aeration and has discussed the subject as follows: 

 The effects of wet and acid soils upon plant behavior have long con- 

 stituted a puzzling problem. The leading role in such habitats as 

 marshes and bogs has been assigned to various factors, such as 

 acids, bog-toxins, toxic exudates, the absence of lime, and the lack 

 of oxygen. Probably all of these are more or less concerned in the 

 problem, with the exception of the supposed exudates, but the view 

 held here is that lack of oxygen is the cause, and the other conditions 

 consequences or concomitants (Clements, 1916 : 90). The presence 

 of acids and bog-toxins is regarded as the direct result of the activity 

 of the roots and bog-flora under deficient aeration (Cf. Stoklasa and 

 Ernest, 1908 : 55; Livingston, 1918 : 95). 



The absence of lime is apparently a concomitant of acid produc- 

 tion, since the addition of lime to an acid soil either neutralizes the 

 acid or affects the colloidal relations in such fashion as to make the 

 soil agriculturally productive. It is significant, however, that lime 

 is not the only substance that has this effect, since it is also produced 

 by other materials which improve aeration. An acid soil is regarded 

 as unfavorable to plant-growth primarily because of the deficit in 

 oxygen, and consequently also because of the poor development of 

 the micro-organisms that reconvert organic nitrogen into available 

 form. The current assumption that bog-water contains acids or 

 toxins which are in themselves unfavorable to absorption seems dis- 

 proved by the experiments of Bergman. 



Plants may indicate good or bad aeration. The former are natu- 

 rally of little importance as aeration indicators, since their impress 

 is due to some other factor or factor-complex. Aeration indicators 

 proper are correlated with a deficiency of soil-oxygen, and are natu- 

 rally confined to wet soils and water, owing to the inverse relation 

 existing between the amount of water and of oxygen. They may be 

 conveniently arranged in four groups, based upon the kind of 

 response to deficient aeration. In the first two the species have 

 developed adaptations which enable them to live so successfully in 

 swamps and bogs that the habit is now obligate for the majority of 

 them. The species of swamps regularly possess a special aerating 



