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root and top growth were seriously affected. Cannon (24) also reported 

 that root growth (with corn) was below normal at oxygen concentra- 

 tions of less than 10 per cent. 



Since the carbon dioxide content of the soil atmosphere increases as 

 the oxygen supply decreases, and carbon dioxide in rather high concen- 

 trations has an apparently toxic effect on roots, it has been suggested 

 that the poor root growth associated with poor aeration is a result of 

 carbon dioxide toxicity rather than lack of oxygen. Vlamis and Davis 

 (7^) found that when barley and tomato plants were exposed to carbon 

 dioxide, a lethal effect was produced. Chang and Loomis (27) bubbled 

 air, nitrogen, and carbon dioxide through culture solutions and found 

 that carbon dioxide reduced both water and nutrient uptake. They also 

 stated that toxic to slightly toxic concentrations of 10-20 per cent carbon 

 dioxide are probably more commonplace in soils than limiting concen- 

 trations of 1-2 per cent oxygen. Parker (55), however, concluded that 

 the carbon dioxide content of the soil is not important in influencing 

 the absorption of inorganic nutrients by plants. Hoagland and Broyer 

 (59) and Arrington and Shive (4) reached a similar conclusion for nu- 

 trient solutions. It thus appears that the main effect of carbon dioxide 

 in most soils is that, when present in large amounts, oxygen tends to be 

 deficient and, through pH changes, the solubility of certain nutrients 

 may be adversely affected. 



It seems amply apparent from the foregoing material that deficient 

 oxygen in the soil would certainly limit the normal growth and exten- 

 sion of roots and development of root hairs and, hence, affect the avail- 

 ability of plant nutrients. Under specialized conditions, as in the green- 

 houses, otherwise normal plants are sometimes found growing with a 

 very small root system where the soil is kept excessively wet and large 

 quantities of fertilizers are used. The plants appear to be dependent 

 upon oxygen dissolved in the irrigation water; this is in a sense a solu- 

 tion culture and in practice it is found that the plants are quite sensitive 

 to any change in their environment. These plants can be grown with 

 limited root systems where the soil functions chiefly as a support. It has 

 not, however, been demonstrated that it is economical or desirable to 

 grow plants in this way. For almost all soils, however, it appears that 

 healthy, productive plants can only be expected when an extensive root 



