314 BOTANICAL GAZETTE [OCTOBER 
mented by an interesting article by LIPMAN and SHARP, giving the 
point at which maximum fixation is reached in a certain soil and 
suggesting that there are really two maxima of fixation with refer- 
ence to moisture content, the one that is most favorable to aerobic 
bacteria, and the other at which the anaerobic forms most actively 
flourish. These tests measured nitrogen fixation under the influ- 
ence of varying percentages of moisture. Of significance also is the 
effect of various periods of drying on the vitality of the nitrogen 
fixing flora. The ability of those organisms to withstand long 
exposure to drying serves somewhat as an index to their activity 
in arid regions and in more temperate climates after long periods 
of drought. . 
It is true also that if any progress is ever to be made in attempts 
to introduce cultures of free nitrogen fixing organisms, Azotobacter, 
for instance, artificially into soils, the persistence of such species 
under exposure to drying will be an important determinant in their 
efficiency. Such phases of the problem as these received attention 
in the present tests. 
Soils which had been in the laboratory 12-18 months, inclosed 
in glass jars and practically air dried, were employed. All of these 
soils had been tested originally for this nitrogen fixing power, 
giving additions of nitrogen ranging from 2.3 to 16.7 mg. per I gm. 
of mannite consumed. Azotobacter had been isolated and grown in 
pure culture from probably 25 of the soils. All of these soils were 
now taken and 5 gm. inoculations were made into 25 cc. of Ashby’s 
medium in order that qualitative examination might be made for 
the organism. Examinations made after 4 days revealed Azoto- 
bacter in only 4 of the soils. Attempts to isolate the organisms 
from these were unsuccessful, although it is probable that if the 
efforts had been repeated the organism would finally have been 
obtained. A number of soils from which it was positively known 
that Azotobacter had been isolated when first brought to the labora- 
tory were then taken, their moisture content brought up to 20 per 
cent, and the soils kept in the incubator room at 28° C. for 3 weeks. 
An opportunity was then afforded for the bacterial flora to revivify 
3 Lipman, C. B., and Suarp, L. T., Effect of moisture content of a sandy soil on 
its nitrogen fixing power. Bor. Gaz. 59:402-406. 1915. 
