160 Mineral Nutrition of Plants 



had reached only 16 per cent (instead of the usual 18.5-19.5 per cent) in 

 an alfalfa-brome-grass plot, and in continuous bluegrass plot the oxygen 

 level had risen only to 14 per cent in 22 hours. Temperatures were low, 

 and it was not thought that production of carbon dioxide in the soil was 

 a significant factor, although it is a remote possibility in these experi- 

 ments. It is surprising to note that renewal rates are considerably lower 

 than published estimates have suggested. 



Direct determinations of oxygen in the soil atmosphere with the 

 oxygen meter have shown very few oxygen concentrations lower than 

 18.5 per cent. Even on experimental plots (52, 55) where soil structure 

 was known to be poor and where crop yields were definitely limited by 

 poor aeration, measured oxygen levels were comparatively high. It was 

 concluded that on most soils aeration is adequate during much of the 

 season when the soil water is at or below field capacity. At times of 

 rains or heavy irrigations soils of good structure drain readily, and 

 normal aeration is re-established before the supply of dissolved oxygen 

 in the soil water is exhausted by the plant. In soils of poor structure, 

 however, aeration may not be so rapidly re-established, and, if the 

 oxygen supply runs out and is deficient for any length of time, the 

 plant may undergo a shock or change which will adversely afTect its 

 future growth and development. Much work needs to be done on the 

 response of plants to intermittent periods of low oxygen supply, to de- 

 termine the extent to which root growth and absorption of nutrients 

 are affected. 



As an example of a situation in which plant growth is definitely 

 limited by physical properties, and to show that these properties are 

 subject to modification, reference will be made to the soil structure 

 rotations of the Ohio Station (§2, 5.?). Various rotations were estab- 

 lished in 1936 on Paulding clay to compare the effects of different 

 cropping systems on soil structure. Summarized results are presented 

 in Table II. It can be seen that "structure building" rotations which in- 

 clude sod crops have effected an improvement in productive capacity 

 of the soil while, with those rotations which do not provide for adequate 

 organic matter return, productivity has sharply declined. 



The changes described have been brought about in a comparatively 

 short time. The effect of fertilizers has been studied on the same plots 



