06 
FIFTEENTH REPORT. 
of Soil Bacteriology,” the answer must be: The aim of Soil Bacteriology 
(aside from the purely scientific interest) is to put the action of the 
soil bacteria in the service of Agriculture, to suppress the detrimental 
species and their action, and to encourage the beneficial bacteria like 
those which accumulate nitrogen, and such as make available those 
compounds of the soil, which are in a state not available for plant 
nourishment. 
To accomplish this, or, in other words, to influence the microorganisms 
in the soils intelligently, presupposes the necessary knowledge of the 
environment of the bacteria, of those factors which are at our command, 
that can be varied as we desire, such as oxygen supply, water, reaction, 
etc. 
Let us now consider the soil and let us treat it from the point of view' 
of a medium for the micro-flora. Soil is composed of three states of 
material, solid material, water, and air; and these three states have a 
marked influence on each other. The soil bacteria are living in the soil 
water; but this soil water is influenced very markedly by the solid 
material and by the air. It is this that makes the soil a difficult 
medium to investigate. I might say here that I know' of no medium that 
is so variable and complex as soil. If we consider milk in this respect, 
the air, and the solid substances, play only a very small role. It can 
also be said that the milk of different cows does not differ materially as 
a medium for the bacteria. On the other hand, we know how large 
are the differences in soils which must necessarily influence their micro- 
flora. 
We encounter still another difficulty if we remember the fact that 
our medium (the soil) is very difficult to sterilize. Only by the action 
of powerful agents are we able to sterilize the soil, in fact, the changes, 
which are necessarily brought about by this sterilization process, are 
so marked that we doubt even if w'e may call this sterilized medium, 
soil. 
This means that in the case of soils we are practically deprived of 
the opportunity of recording the action of single species of microorgan- 
isms. Further soil is especially characterized by enormous surfaces. 
To give an idea how great the soil surface is, I should like to cite the 
work of Alfred Mitscherlich* who came to the conclusion that the 
outer surface of one gram of quartz sand was 1.38 square meters and 
that of one gram of clay w r as 966.7 square meters. These enormous 
surfaces give us an idea of how closely the soil water can be in contact 
with the solid soil substance. 
In regard to the permeability, I regret to say that we have but very 
little trustworthy data. The reason for this is evident, namely, that 
the many values obtained with air dried soil do not permit any con- 
clusions for field conditions. We may sav, however, that the permea- 
bility of the different soils is extremely variable. 
From the foregoing, it is clear that, even in fine tertiary quartz sand, 
which has so great a surface, there is great possibility for action be- 
tween the soil water and Ihe absorbed substances on the soil grains. 
On the question of, “What is the nature of these reactions between 
the soil water and the soil particles,” the answer cannot be very satis- 
factory. Permit me, however, to draw your attention to some experi- 
*A. Mitscherlich- Bodenkunde fur Land und Forstwirte, 1905 p. 49-73. 
