50 
THIRTEENTH REPORT. 
oxygen from the air is more readily used by the organisms than the 
oxygen from the nitrate. 
These examples will suffice to emphasize the previous statement, that 
soil problems must be studied in soil and not in liquid media. Since the 
development of anaerobic bacteria of the soil is greatly increased in 
solutions, while aerobic organisms are suppressed, it is obvious that 
changes taking place in solutions allow of no conclusions whatever in 
regard to changes in soil. We must necessarily expect erroneous and 
even contradictory results if we neglect variations of 700%. Strange 
to say, bacteriologists have, until quite recently, paid no attention to 
this difference. This serious error has been made for more than ten 
years, and a large amount of work done in this period is probably wasted, 
since we do not dare to bring the results obtained into any relation 
with soil problems. 
I have emphasized this problem of the “soil as a culture medium” 
for two reasons. The one is that our laboratory is dealing with this 
special problem, the other is the urgent need of this work. How can 
we expect to make progress in soil bacteriology if we neglect factors 
of the most vital importance for microbial development? 
Soil is such a complex mass of organic and inorganic, soluble and 
insoluble compounds, containing so many different organisms that we 
cannot possibly oversee all the changes taking place at once. It will be 
absolutely necessary to reduce the number of unknown factors in soil. 
It will become necessary to study the development of pure cultures in 
soil lnd'ore we can think of dealing with the natural microbial flora and 
fauna of soils. It will require a large amount of careful and possibly 
discouraging work to study the peculiarities of soil as a culture medium, 
but it offers a chance for fundamental, and, I dare say, classical work 
in soil bacteriology. 
Processes other than aeration will probably play a role in the 
microbial development in soil, since it is believed by some chemists that 
even chemical reactions are different in soil from solutions. 
Cameron stated last year (Journal of Physical Chemistry, Vol. 14, 
p. 402 (1910) “We should expect that many reactions would take 
place quite differently in the soil from the way they would in a beaker 
or flask. This fact has been generally overlooked or ignored and is 
probably the explanation of many of the apparently anomalous results 
hitherto reported in chemical investigations of soils. Enough is 
known to justify the statement that the chemistry of the soil need not be 
and probably is not, the chemistry of the beaker.” 
And still, another factor must be considered as possibly influencing 
microbial development in natural soils. The publications of the Bureau 
of Soils have demonstrated that there are organic compounds in soil 
which retard plant growth. The same or similar compounds may retard 
bacterial growth. Investigations have never been made along these 
lines, but the possibility must be considered. 
One other problem must be mentioned here which has been consid- 
ered so little as yet that we cannot even say whether or not it is im- 
portant to soil fertility. It is the relation of microorganisms to the 
physical structure of soils. The formation or destruction of humus will 
certainly influence the physical qualities of soils. But even aside from 
that, soil is changed physically by microorganisms. A root, a leaf, a 
piece of straw in soil will decay and the space previously occupied by 
