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of ions through the individual aggregates appeared to control the rate 

 of exchange, for macroscopic velocities in excess of 0.004 cm - P er second. 

 The two soils did not behave identically and, for soils in general, much 

 individuality of behavior seems probable. 



Rates of internal diffusion were found to limit the exchange rates 

 with the fine aggregates also, provided that the macroscopic velocity 

 of the permeating fluid was made sufficiently great (e.g., 1-2 cm./sec). 

 Such high velocity, however, is unlikely to be of direct interest in prob- 

 lems of plant growth in soils. 



From these experiments it appears that ionic diffusion rates within 

 aggregates may possibly affect the ready availability of exchangeable 

 cations, particularly if the aggregates are large, impenetrable by roots, 

 and not easily dispersible. 



ROLE OF SOIL AERATION IN NUTRIENT UPTAKE 



Soil aeration not only affects the extent and character of root growth, 

 but is also of extreme importance through the effect oxygen has upon 

 the assimilation of nutrients by roots. Much of the more conclusive evi- 

 dence again is to be found from studies in solution cultures, primarily 

 with excised root systems. Since this work will be the topic for another 

 paper on this symposium, it will not be reviewed in detail here, but a 

 few papers will be cited to indicate the essentiality of oxygen in absorp- 

 tion. The work of Steward and his colleagues (65) showed that, for 

 tissues, aerobic respiration supplies the energy necessary for salt absorp- 

 tion against a concentration gradient. The extensive studies of Hoag- 

 land and Broyer ( 39, 40) proved that salt accumulation by roots is also 

 dependent upon aerobic metabolism, and oxygen is one of the indis- 

 pensable requirements for salt accumulation (movement of salt against 

 a gradient) by excised barley roots. These studies were made on assimi- 

 lation of potassium, halide, and nitrate. They also showed that a rela- 

 tively high concentration of carbon dioxide is required to greatly de- 

 press salt accumulation. Where oxygen had been carefully excluded, 

 they did not observe accumulation of salt against a gradient. The recent 

 work on this problem is excellently summarized by Hoagland (41). 



In work with growing tomato plants in solution cultures, Arnon and 

 Hoagland (_?) found that roughly 1.4 times as much each of potassium, 



