Soil Changes Produced By Micro-organisms. 23 
of years is common knowledge to every experienced farmer. The 
available potash and phosphoric acid may have increased some, but 
the greatest difference observed is in the amount of nitrogen 
present. At the end of the fallow period, we should expect to find, 
and do find, more nitrate nitrogen due to nitrification, but far more 
important than this is the appreciable addition to the total nitrogen. 
Now the air with its 79 per cent, of nitrogen could easily supply 
this amount provided some means were at hand for transferring 
it from the atmosphere in a gaseous form to the soil in a combined 
form. 
Many chemical theories were formulated and expounded during 
the third quarter of the nineteenth century to explain this phenom¬ 
enon, but all failed when put to practical tests. For a long time it 
had been observed that plants of the legume family, represented by 
alfalfa, clover, peas, and beans, made good yields irrespective of 
the nitrogen in the soil. Not only was this true, but the soil itself 
was benefited by the legume, as was indicated by the crop that 
followed. This led to an analysis of the soil which showed very 
clearly that it actually contained more nitrogen after it had grown 
a crop of peas than before. In other words, the increase in nitrogen 
was, in some way, related to the life processes of the leguminous 
plant. Finally, the mystery was solved, and it was proven con¬ 
clusively that the responsible agents were specific soil bacteria 
which both independently and when associated with legumes held 
the key to the great storehouse of atmospheric nitrogen, and that 
through their activities alone, this supply of nitrogen could become 
available for the higher plants. 
We recognize two distinct types of activity and two different 
groups of bacteria in the fixation of atmospheric nitrogen. In the 
one, the micro-organism performs the work by itself; in the other, 
the combined action of germ cell and legume is required. The 
former is spoken of as non-symbiotic and the latter as symbiotic 
fixation. 
N on-Symbiotic Fixation. 
In the early experiments which were conducted by Berthelot 
in 1885, to determine the cause of the increase of nitrogen in fallow 
ground, two lots of soil were obtained from the same source. One 
portion was sterifized by heat to destroy all life, and the other was 
kept in its normal condition. v Both samples were exposed side by 
side to the same conditions and both were treated as nearly alike 
as possible. After a given length of time had elapsed, the total 
nitrogen was determined in each, and it was found that the heated 
