RESPIRATION AND OXYGEN. 103 



"3. The bad growth of sal seedlings in this soil is correlated with an accumu- 

 lation of carbon dioxid in the soil-solution and a low oxygen-content, and this 

 possibly explains the evil effects of bad aeration. Further work, however, 

 is required to prove this and also to decide the relative importance of carbon 

 dioxid and oxygen, respectively. 



"4. Liming this soil, immediately before sowing, has an injurious effect 

 upon sal seedlings, and during the rains, soil which has been thus limed 

 appears to contain more carbon dioxid and less oxygen than the unlimed soil. 

 It seems possible that this may be due to accelerated bacterial activity. 



"5. As carbon dioxid is rapidly dissipated and a deficiency of oxygen made 

 good under the ordinary conditions of water cultures, it is not easy to prove 

 the effect of varying quantities of these gases on plants grown in cultures. 

 For the same reason artificial aeration of such cultures may not show any 

 beneficial result. 



"6 As sal seedlings can be successfully grown in water cultures, the inju- 

 rious effect of bad aeration is not due to water as such This probably 

 explains the fact that sal can grow on the banks of the rivers or even of stag- 

 nant lakes, in which the water is kept well aerated by exposure to the air or 

 by the presence of green aquatic plants." 



Bernbeck (1914 : 26) has pointed out the importance of fresh 

 soil-air for the growth of tree roots and emphasized the relation of 

 the amount of air in the soil to the porosity. Graves (1915 :213) 

 has studied a disease of coniferous seedlings growing in clay seed- 

 beds. The disease caused most havoc during the wet months, while 

 many cases of recovery occurred in the drier months. In porous 

 soil, in the same nursery, the disease has never been known to occur, 

 and he concludes that it is due to a lack of oxygen in a soil saturated 

 with water. 



Howard and Howard (1915 : 19) have concluded that the so-called 

 disease of Java indigo in India is due to long-continued saturation 

 of the soil, which leads to the death of the young absorbing roots, 

 consequent leaf-fall, and more or less complete wilting. In dealing 

 with soil ventilation (1915 2 : 35; 1915 3 : 11), they have discussed the 

 relation between aeration and manuring, green manuring, fallow- 

 ing, packing of the surface soil, earth mulches, rice cultivation, grass 

 effects, simulated diseases, peach yellows, surface crusts, and the 

 saving of irrigation-water. Of especial interest are the so-called 

 natural aerators, plants such as Cajanus indicus, Trifolium resupi- 

 natum, "busunduk," and alfalfa, which serve to break up the soil 

 by reason of deep-seated tap-roots or large laterals. They empha- 

 size the fact that crops differ greatly in the air requirement of their 

 roots. Gram is cited as an example of a crop that requires a great 

 deal of air and but a moderate amount of water. Hence great care 

 must be taken to secure and maintain the proper relation between 

 air and water in the soil. 



Howard (1916) has discussed the improvement in the aeration of 

 field soils under surface drainage, and has given a resume of the 

 whole subject of aeration in another bulletin (1916 2 ). In this it is 



