Wadleigh and Richards 427 



moisture tension at the six-inch depth dropped from 700-800 cm. of 

 water to 11-15 cm. and remained that low for 4 or 5 days after irri- 

 gating. During this period of low moisture tension, oxygen content of 

 the soil atmosphere dropped from about 20 per cent to 5 per cent. 

 However, a concomitant change in carbon dioxide content was not 

 noted. As soon as 100 cm. of water tension developed following irri- 

 gation, there was observed a rapid increase in oxygen content of the 

 soil atmosphere. The oxygen content of soil air at the 30-inch depth 

 also markedly reflected the irrigation cycles, whereas that at the 96-inch 

 depth was little affected, oxygen content persisting continuously at 15- 

 17 per cent. However, the carbon dioxide content of the soil air at the 

 greater depths maintained itself at about 5 per cent. These data illus- 

 trate very well the interrelationship between soil moisture content and 

 composition of the soil air. They are especially striking since they were 

 obtained on a relatively coarse-textured soil— Indio very fine sandy loam. 

 Boynton (/j) found that fluctuations of ground water in relatively 

 heavy soils was usually fairly well correlated with fluctuations in oxy- 

 gen and carbon dioxide percentages of the soil air, particularly in the 

 strata of soil just above the water table. Interestingly enough, some- 

 times a month or more elapsed between the disappearance of ground 

 water and the rise of oxygen level to a maximum. Boynton and Reuther 

 (14) recorded oxygen percentages in the soil air as low as 0.1 in the 

 second foot of a Dunkirk silty clay loam under an apple tree during 

 March and April. On subsidence of the water table during the summer, 

 the oxygen content at this depth rose to 17-19 per cent, whereas the 

 carbon dioxide content remained at about 3-4 per cent throughout the 

 year. In the fourth foot, the carbon dioxide content of the soil air re- 

 mained 8-10 per cent throughout the summer. Excess soil moisture 

 conditions soil aeration which, in turn, affects mineral nutrition. 



The essentiality of adequate aeration for the development of healthy 

 roots and vigorous plants of most species has been emphasized many 

 times both in soils (no, 7, 49, 86, 39, 26, 5, 76) and in solution cultures 

 (75, 48, 109, 41). Numerous studies by Steward and co-workers (113, 

 114), Hoagland and Broyer (60), Lundegardh (78), and Robertson 

 and Wilkins (/05) have amply shown that the rate of nutrient entry 

 into absorbing tissue is conditioned by the rate of respiration of the 



