T 34 
Journal of Agricultural Research 
Vol. XXV, No. 3 
surface, but it is surely not generally proportional to the oxygen content 
of the soil in general at the given depth; a soil containing very little 
oxygen might still deliver oxygen to the absorbing surface at a considerable 
maintained rate, while a soil containing much oxygen might soon become 
depleted near the absorbing surface so that its maintained rate of delivery 
would be low. Assuming that roots require an oxygen supply from the 
soil, the oxygen condition that determines whether they shall be healthy 
or unhealthy is therefore not to be defined in terms of the oxygen content 
of the soil at the various depths where the roots occur. The soil feature 
that determines the health of the roots, as far as oxygen is concerned, 
must be the oxygen-supplying power. It should be noted that, like other 
environmental conditions, this dynamic feature of the plant's subterra¬ 
nean environment ought to be a limiting condition only when its value is 
lower than the necessary rate at which oxygen absorption must proceed 
if the plant is to remain healthy. As long as the soil is able to supply 
oxygen more rapidly than it is required by the roots, it should make no 
difference—other conditions being adequate for health—how great the 
oxygen-supplying power may be. 
It seems that ecology and agricultural science would be advanced if 
we might be able to study and compare the oxygen-supplying powers of 
field soils at various depths, down to the lower limit of penetration by 
the roots of plants growing in them. The dynamic soil feature here 
emphasized has apparently not yet attracted the attention of ecologists, 
foresters, and students of crop plants; indeed, it seems only to have been 
barely mentioned in the literature thus far. It was somewhat surprising 
to find no mention of this dynamic consideration in such a thorough 
review of the literature of soil aeration as that recently made available 
by Clements* *s 3 excellent monograph on this subject. In the Year Book 
of the Carnegie Institution of Washington for 1921 there is a report by 
W. A. Cannon, 4 in which he says: 
It is the rate of supply and not the partial pressure of the gas (in the soil air) that 
is important. 
Such studies and comparisons as those just suggested can not, of course, 
be begun until some suitable method has been devised for measuring 
the oxygen-supplying power of the soil, and the work here reported was 
undertaken for the purpose of testing certain suggested methods that 
seemed to have some promise in this direction. The results herein 
reported on the oxygen-supplying power of the soil were obtained dur¬ 
ing January and February, 1922. 5 
METHOD. 
Any method for measuring the power of the soil to supply oxygen to 
an absorbing surface must fulfill two conditions: (1) The absorbing 
apparatus must allow oxygen from the surroundings to enter by diffu¬ 
sion and not by mass streaming (since there is no mass streaming of 
oxygen into plant roots), and the absorbing surface must not alter 
significantly, as to its ability to absorb oxygen, when the surrounding 
* Clements, Frederic E. aeration and air-content, the r6le of oxygen in root activity. 183 p. 
Washington, D. C. 1921. Bibliography, p. 163-183. (Carnegie Inst. Wash. Pub. 315.) 
* Cannon, W. A. root-growth in relation to a deficiency of oxygen or an.Excess of carbon 
dioxid in the soil. In Carnegie Inst. Wash. Yearbook 20 (1921), p. 48-51. 1922. 
6 These studies were carried out in the laboratory of plant physiology of Johns Hopkins University under 
the general direction of Dr. M. B. Waite, pathologist in charge, Office of Fruit-Disease Investigations, 
Bureau of Plant Industry, U. S. Department of Agriculture. 
