Page and Bod man 14c 



in their experiments were definitely limited by soil structure and aera- 

 tion. Trogdon (68) found a highly significant correlation between yield 

 of corn and air pore space in the soil, irrespective of the amounts or 

 forms of fertilizer applied, provided essential elements were not lacking. 

 Cannon and his co-workers have conducted outstanding research on 

 root-aeration relationships. Cannon and Free (26) stated: "Increased 

 air supply to roots if not excessive favors root branching and probably 

 accelerates the rate of root growth." They further suggested that aera- 

 tion may be of equal importance ecologically with temperature and 

 water supply. The effect of low oxygen levels and temperature on the 

 growth rate of corn is brought out in Table I which gives some of Can- 

 non's (24) results. 



TABLE I 



Effect of Oxygen Levels and Temperature on Rate of Growth of Corn 



(Cannon 2^) 



Rate of Growth of Corn With the Following Oxygen 

 Levels in Soil Air 

 Temp. 3 per cent 3.6 per cent 10 per cent 



normal 



1 8° C. 1/3 normal 2/3 normal 



20 1/5 normal 



3°° i/ J 6 normal 1/3 normal 9/10 normal 



It can be seen from Table I that there is a significant influence of 

 temperature upon the relation of roots to oxygen. The effect of low 

 oxygen levels is most pronounced at higher temperatures so that for 

 best growth rates during hot weather, soil aeration must be excellent. 

 Cannon (22,25) a ^ so studied several species of plants differing in 

 natural habitat and concluded: (a) complete absence of oxygen resulted 

 in cessation of root growth; (b) many species can grow in an environ- 

 ment with as little as 0.5 per cent oxygen, but only very slowly; (c) rate 

 of supply rather than partial pressure of oxygen governs growth rate; 

 (d) when the supply of oxygen was too low to permit normal growth, 

 the growth rate varied inversely with the temperature; (e) at constant 

 temperature, growth varies directly with partial pressure of oxygen; 

 and (/) there is a critical oxygen concentration for growth, but the ex- 



