100 AERATION AND AIR-CONTENT. 



ference increases with increasing water-content. The soil of fields 

 in the latter condition is significantly more permeable to air than one 

 in the powdery condition. 



Hartig (1894 : 275) pointed out the importance of oxygen for the 

 roots of trees, and stated that the latter die from asphyxia if excluded 

 from a constant supply. Physiological root-rot of pines and spruces 

 is due to lack of soil-air, owing to the density of the soil or to its 

 water-logged condition, and a similar root-rot occurs in plants 

 grown in glazed pots. The remedy for both of these lies in better 

 aeration of the soil. In beech woods the failure of natural regenera- 

 tion is often due to poor aeration resulting from the thick layer of 

 humus. 



Warming (1895 : 96; 1909 : 43) regards soil-air as of the most 

 fundamental significance to plants, since roots and underground 

 shoots, like all other living parts, require oxygen for respiration. 

 Plants adapted to ordinary soils are suffocated in very wet soils, and 

 this results in alcoholic fermentation, followed by death and putre- 

 faction. Soils poor in oxygen exhibit a different type of decompo- 

 sition, and they become ''sour" in consequence of the formation of 

 great quantities of humous acids. The production of acid humus in 

 the forest leads to the exclusion of air and the death of the trees. 

 Ramann (1895, 1911) has likewise insisted that aeration is one of 

 the most important of soil processes for the plant, since it has to do 

 both with the access of oxygen and the removal of excessive carbon 

 dioxid. 



Mangin (1896 : 67) made a comprehensive study of the relation 

 of the amount of CO2 and oxygen in the soil-air about the roots of 

 trees to the well-being of the latter. He found that soils which are 

 packed are less aerated than other soils, and that the grills placed at 

 the base of trees in porous soils were sufficient to assure good aeration. 

 In compact soils or in consequence of watering, which makes the soil 

 of the basin but slightly permeable, the renewal of the air by grills 

 did not suffice to prevent the accumulation of CO2 under the bitumen 

 in quantities sometimes considerable. The watering of the basins 

 presented serious difficulty in consequence of the compacting, which 

 diminished the permeability of the soil to both air and water. While 

 the aeration of the soil was very good in many places in the prome- 

 nades, there were other places where the amount of CO2 reached 8, 16, 

 and even 24 parts per 100, while the oxygen was reduced as low as 

 15 or 10, or even 6 parts. Since CO2 in the soil noticeably decreased 

 growth, all of the trees found in the badly aerated soil slowly perished 

 because of it, as well as owing to the lack of oxygen. 



Deh^rain (1896 : 468) reached the following conclusions with ref- 

 erence to the effect of tillage on aeration: Untilled soil is very well 

 aerated. It inclosed as much air in the prairies and woods as when 

 covered by spontaneous vegetation. Although the quantity of air 



