162 AERATION AND AIR-CONTENT. 



Many successional stages and climaxes have been under detailed 

 observation in Nebraska and Colorado since 1896, without the slight- 

 est evidence that toxins are in any manner concerned in their con- 

 dition. Some of these are more luxuriant than when first seen, and a 

 close study of their growth from year to year has shown that it varies 

 only in relation to the rainfall and the resulting water-relations. It 

 seems certain that most climaxes have occupied their habitats for 

 thousands of years, or even longer, and that their present growth 

 and composition make the depressing effect of toxins unthinkable. 

 In short, they extend the Rothamsted results with soils continuously 

 cropped from 60 years to thousands of years. 



Soil toxins are probably to be definitely related to deficient aera- 

 tion and to anaerobic conditions, as has been indicated by Schreiner, 

 Hall, Russell, and others. This is also shown by the fact that they 

 are readily oxidized, and soon disappear under proper tillage. Hence, 

 they appear to be due to essentially the same conditions and processes 

 as obtain in bogs, the relationship being especially well exhibited by 

 muck soils. In both, the primary causes of toxicity are the direct 

 lack of oxygen and its indirect effect in permitting the accumulation 

 of carbon dioxid in harmful amounts and in producing injurious 

 organic acids and other compounds. In many cases probably the 

 first two alone are concerned, but in sour soils and muck soils at least, 

 all of them must have a part, though the lack of oxygen plays the 

 primary role. Since carbonic and other acids are the products of 

 respiration under such conditions, a considerable part of soil acidity 

 may be ascribed to them, though it must be recognized that toxic 

 effects may arise from acidity otherwise produced, as shown in the 

 preceding section. In conclusion, the present facts appear to war- 

 rant the statement that organic toxins are excreted by roots or pro- 

 duced in soils only as a consequence of the anaerobic respiration of 

 plant roots and of micro-organisms, and that inorganic toxins may 

 arise as a result of chemical processes or of adsorption. 



