936 
Journal of Agricultural Research 
Vol. XXIV, No. II 
one essential condition to be fulfilled, in rendering a soil capable of 
supporting Azotobacter. This has been accomplished experimentally 
in a number of different ways. Perhaps the simplest and most eco¬ 
nomical one is the addition of sufficient lime to reduce the hydrogen- 
ion concentration to a point below i X io“^. Attention has also been 
called (j) to the ease with which this can be done in the laboratory. 
In a forthcoming publication it will be shown that the same is true 
under natural soil conditions. The most acid soil that has been en¬ 
countered locally required approximately 18,000 pounds of calcium 
carbonate to reduce the acidity to below Ph 6.0. Three years ago 
sufficient lime to accomplish this was added under natural conditions 
accompanied by inoculation both with soil containing Azotobacter and 
with cultures grown in the laboratory. These plots still contain a vigor¬ 
ous Azotobacter flora and will fix, in laboratory media, two to three times 
as much nitrogen as will adjacent, similarly inoculated, unlimed plots 
from which the Azotobacter disappeared within a few days following 
inoculation. 
It is not the purpose of this paper to advocate the use of lime and 
Azotobacter inoculation solely as a means of aiding the maintenance of 
the nitrogen supply of soils. For, while it is believed that this factor 
alone would justify the expense, the economic phases of the proposition 
have not been sufficiently investigated. 
Experiments are now under way that promise to solve this phase of 
the problem locally. However, it is generally agreed that permanent 
agriculture is impossible without a soil sufficiently supplied with lime. 
Nature has provided ample means for maintaining the nitrogen supply 
of native soils. The American farmer has so ignored nature’s methods 
that many soils have become depleted of their nitrogen supply and are 
productive only when supplied artificially with nitrogen at an enormous 
expense. One of nature’s methods is through the agency of different 
types of nitrogen-fixing bacteria. One essential requirement of these 
organisms is a low hydrogen-ion concentration in the soil. While the 
quantity of nitrogen fixed by these organisms might not alone justify 
^e expense of maintaining a low degree of acidity, this factor, coupled 
with the other recognized physicochemical benefits to the soil and physio¬ 
logical benefits to the growing plants, would unquestionably justify 
such a practice. The possible influence of adequate liming upon the 
nitrogen economy of the soil is mentioned as an added stimulus to the 
use of lime. 
Another point to be noted is the general similarity of results, whether 
based upon the electrometric or the colorimetric method of determining 
the hydrogen-ion concentration of the soil solution. The individual 
determinations frequently do not agree as closely as might be desired. 
Yet the conclusions, so far as this study is concerned, would be identical 
regardless of data upon which they were based. In a general way the 
farther removed from neutrality, the wider the variation between the 
two methods. The electrometric method usually recorded a higher or 
lower hydrogen-ion concentration, as the case might be, than the colori¬ 
metric method if the soil were appreciably acid or alkaline. It is believed, 
however, that the general agreement between the two methods is suffi¬ 
ciently close to justify the use of the much quicker and less expensive 
colorimetric method as an aid in the solution of many soil problems, par¬ 
ticularly where the initial expense prohibits the purchase of the necessary 
expensive electrometric apparatus. 
