48 
THIRTEENTH REPORT. 
which prey upon The essential soil bacteria reducing their numbers and 
consequently soil fertility. 
This is, briefly told, a bacteriologist's conception of soil. The object 
of this paper was to discuss soil problems rather than facts. It is, of 
course, impossible to enumerate in detail all the many points of interest 
in soil bacteriology. I shall try to present the most prominent problems 
which demand immediate attention. This introduces a personal ele- 
ment into the discussion, since not all bacteriologists value the same 
problems alike. If I speak of important problems, I mean those that 
are considered to be of vital interest by the laboratory which I have the 
honor to represent. 
A problem that is at present taken up from different sides is that of 
the humus. The formation of humus has been studied verv little, de- 
spite its significance, because of the insufficient chemical technique of 
its analysis. Efforts have been made recently bv the Bureau of Soils 
to obtain a more definite idea of at least some of the many humous 
bodies, and it can be hoped that before very long humus will be known 
as a fairly well defined group of organic compounds. The bacteriolo- 
gist's task is to find the organisms that form humus, and to study their 
habits and their food requirements. This knowledge will enable us to 
increase humus formation where it seems necessary and to prevent it, 
when desirable. The humus problem also includes the destruction. 
The organisms breaking down humous compounds to ammonia are very 
important because while not available itself, humus yields decomposition 
products which are the most valuable plant food, namely ammonia and 
nitrates. By obtaining a perfect knowledge of the humus-forming and 
the humus-destroying organisms, we gain control of the humus content 
in soil. 
The peat problem, or more definitely the use of peat as farm soil or 
as fertilizer is nothing but a side line of the great humus problem. 
Though there is undoubtedly a great difference between peat and humus, 
it seems quite probable that the solution of one problem will make the 
solution of the other an easy task. 
Another problem is the constancy of the number of bacteria in soils. 
This factor may not seem very important to the superficial observer, 
but it has doubtless close relations to soil fertility. In decaying organic 
matter, in fermenting liquids, in sewage, and in milk the number of 
microorganisms will first increase rapidly, then decrease and in a com- 
paratively short time most of the organisms are dead. In a given soil 
the number of bacteria is practically constant. Slight fluctuations are 
caused by seasonal influences, by rain and drought, by fertilizers, but 
the soil soon regains its original number of microorganisms. The old 
explanation of the constancy as an equilibrium established betweeu the 
various kinds of microorganisms is not satisfactory. If such an equili- 
brium were possible at all. it would be found in liquids too. But that 
is not the case. There may be an equilibrium between bacteria and 
protozoa, but we have no accurate knowledge as yet of soil protozoology. 
The constancy of the bacterial flora in soil is very important because 
it renders possible the immediate decomposition of organic matter en- 
tering the soil. In solution, where the organisms die after having acted 
for a short time, the decomposition soon comes to a standstill. In soil 
this does not seem to be so, at least, the limit is much wider. The 
