102 AERATION AND AIR-CONTENT. 



and was likely to establish itself only on well-drained soil not subject 

 to water-logging. 



Balls (1912 ; 38) found that the roots of the cotton plant in Egypt 

 were locally asphyxiated in water-logged soil, and in a few weeks even 

 the stout woody roots were not merely dead, but also decomposed. 

 When the depth of available soil was decreased by raising the 

 water-table and thus asphyxiating or killing the lower part of the 

 root -system, greater absorption was demanded of the surface roots. 

 Excavations of a root-system which had been partially submerged in 

 subsoil water showed all the original tap-roots and branches dead 

 from 160 cm. to the maximum, 220 cm. This level coincided with 

 the maxim.um height of the water-table, which had been maintained 

 for 10 days at the end of September. Alongside of the brown and 

 partly decomposed roots were new white roots in abundance, which 

 ended at various depths up to 210 cm. These new roots were all 

 found to arise from laterals which had not been reached by the 

 water-table. With the fall of the latter, these healthy laterals had 

 developed hundreds of rootlets that grew downward as the water 

 receded. 



Harrison and Aiyer (1913 : 106) have reached the conclusion that 

 the surface film of algae is the chief agent in the aeration of the 

 roots of the rice crop. The oxygen evolved by the algal film is 

 dissolved by the irrigation water to produce a highly aerated solution. 

 In undrained soils this can not enter the soil, with the result that the 

 roots are congested near the surface, thus Hmiting the area of root 

 action. In drained soils the aerated water is carried downward, and 

 the roots consequently penetrate to greater depths. The mass of 

 soil subject to absorption is increased and the crop is correspondingly 

 benefited. Too rapid drainage hinders the formation of the algal 

 film and lessens the consequent aeration. The optimum rate of 

 drainage for all swamp paddy soils is a comparatively slow one. 

 This is due to the fact that aeration by atmospheric oxygen is less 

 effective in promoting root aeration than that by the aerated water 

 draining through them. The use of green manures in drained paddy 

 soils promotes the activity of the algal film and thereby increases 

 the aeration of the roots. 



The studies of Hole and Singh (1914 : 10) upon aeration in forest 

 soils in India show that lack of oxygen is a factor of great impor- 

 tance and wide extent. The general summary of their results is as 

 follows (101); 



"1. The present experiments have confirmed the results previously ob- 

 tained regarding the very injurious effect of bad aeration on the growth of 

 sal seedlings in the local forest soil. 



"2. When water is long held in contact with this soil, which is the case 

 under conditions of bad aeration, it becomes heavily charged with carbon 

 dioxid and impoverished as regards its supply of oxygen. 



